Capabilities - I feel the key to understanding where and when to apply a unit in any given situation is informed by knowing their full capabilities off by heart; here is a look at what this unit is capable of.
Designed primarily to provide mobile artillery-scale firepower to Skitarii forces, Onager Dunecrawlers are able to devastate multiple distinct types of enemy based primarily on their weapon selections. Strength 10 AP1 blasts, guns that increase in power at the cost of spread (a shotgun in all but name) and multi-layered dedicated anti-aircraft weaponry provide Skitarii with strong long-ranged support that appropriately supplement their already considerable firepower and even eliminate possible deficiencies in the rest of the army list. Their melee capabilities are understandably lacking as a result of this ranged focus; a single Initiative 2 Attack with Weapon Skill 3 and Strength 5 may as well be scraping the bottom of the barrel for Walkers in 7th Edition - of note here is that they can be armed with what amount to Power Fists but these are expensive and don't solve the Onagers' inherent lack of attacks. Due to this deficiency they are highly vulnerable to being tied up in combat and are unlikely to survive long against dedicated close assault forces, though otherwise they are actually rather survivable given their price point. AV12/12/11 with three hull points is fairly average for the Onagers' unit classification, but it is their Emanatus Force Fields that truly sustain them; an exponentially increasing invulnerable save based on how many models are in a Dunecrawler squadron, these embedded shields allow Onagers to resist a great deal of punishment from all sources in any phase of the game. This defensive benefit exclusively rewards investing in multiple Onagers on a unit-by-unit basis while further amplifying the need to protect them from assaults with nearby supporting Skitarii forces.
The Crawler special rule allows Onagers to ignore both Difficult and Dangerous terrain types so as to freely move through and over any and all obstacles without impediment, but this does come at the cost of the walkers being incapable of performing Run moves. Coupled with the natural Relentless nature of vehicles, this permits Onager Dunecrawlers to negotiate a battlefield without hindrance while setting up good firing positions and making the most of their heavy firepower. These plodding machines of war additionally feature Broad Spectrum Data-Tethers and Searchlights to simultaneously provide other Skitarii with Leadership bonuses upon activation of Doctrina Imperatives and ignore the effects of Night Fighting against any single target of your choice. Logically, the Onagers themselves also benefit from the various Doctrina Imperatives and can thus benefit from up to three turns of dramatically increased accuracy on top of their already acceptable Ballistic Skill 4, while the melee improvements are serviceable should they be required. Able to survive a great deal of punishment - especially in a full squadron of three models that coalesce their Force Fields - and wreak havoc on enemy battle lines or flyer squadrons alike, Onager Dunecrawlers are very much a crucial element of any Skitarii force and can supplement an army with a wide variety of devastating weapons all backed by efficient pricing.
How to Equip Them - Outfitting a unit is often even more difficult than determining the number that you employ, so having appropriate context for each choice is key; this rounds out the most optimal ways to equip a squad.
An important consideration to using Onager Dunecrawlers is that maximizing their potential usage normally involves fielding two or three of them in a single unit due to their Emanatus Force Fields and Field Harmonics rules. This brings up a particular issue; individual Onagers in a squadron are incapable of selecting different targets and are instead forced to combine their fire at a single enemy unit, rendering mixed weapon selections less useful than they otherwise could be - this is especially the case because most of the guns available have a unique purpose. The stock Eradication Beamer functions much like a modified Conversion Beamer that increases in both Strength and AP at close ranges for a powerful anti-tank shot but is instead a light infantry suppression large blast source at great distances. The twin-linked Heavy Phosphor Blaster strips cover saves from those unlucky enough to be its prey, though it packs enough punch to easily harm 3+ armored foes of varying Toughness values between one and seven or, handily, most light vehicles. An Icarus Array functions as a three-in-one amalgamate weapon system that is disgustingly effective against aircraft of all shapes and sizes - be they Skimmers, Flying Monstrous Creatures or Flyers - but is of little use against anything else. Lastly, the Neutron Laser obliterates elite infantry and Toughness 5 models with multiple wounds through its single Strength 10 AP1 small blast that even penalizes the Initiative values of those struck by its energies, inflicting the Concussive penalty upon them.
Mixing the Eradication Beamers and Neutron Lasers works well enough as the former supplements the anti-tank capabilities of the latter while providing additional anti-infantry clearing through blast type shots. Icarus Arrays can work well alongside Heavy Phosphor Blasters and - normally - little else given that blast weapons are incapable of being fired as Snap Shots without the accompanying Skyfire special rule, while the twin-linked high Strength shots offered by the Heavy Phosphor Blaster can add that minimal extra punch required to destroy aircraft and this is especially the case where Skimmers are concerned. Armies that do utilize Skimmers in abundance can allow for effective usage of mixed Icarus Array and Neutron Laser or Eradication Beamer equipped Onager Dunecrawlers, but otherwise the latter two weapon types tend not to function well alongside the former for efficient usage of each constituent part. I rate all of the weapon options highly for varying reasons as each is useful in their own right - the Eradication Beamer for its multi-purpose role, the Heavy Phosphor Blaster for its accuracy and support functionality, the Neutron Laser for its raw devastating damage potential and the Icarus Array for its near incomparable anti-aircraft shooting - but I tend to find the Neutron Lasers and Icarus Arrays are the most common and useful. Skitarii forces can struggle to deal with massed elite infantry or well armored models with multiple wounds featured in squads, while Onagers are a staple of secondary or tertiary detachments due to the sheer effectiveness and efficiency of Icarus Arrays to complement another factions' aerial countermeasures.
Additional investments can be made into lesser systems on individual Dunecrawlers with varying levels of success, including Cognis Heavy Stubbers to add cheap firepower and charge-resisting guns, Smoke Launchers for a one-use-only defensive benefit that is generally unnecessary given their Emanatus Force Fields and even Mindscanner Probes that function as area-of-effect defensive grenades. Most interesting and expensive of all though are the Cognis Manipulators that add what amount to Power Fists to the Onagers' repertoire, granting them Strength 10 AP1 attacks in an assault to greatly improve their otherwise meager assault capabilities. However, these are noteworthy less for the impact they provide and more because they confer the It Will Not Die special rule on the user; these are most useful for Onager squadrons that can utilize powerful 4+ invulnerable saves and varying model placement on a turn-by-turn basis to "spread" damage around and regenerate hull points lost. The additional expenses each of these various upgrades provide may be too much to swallow for some but they tend to justify their worth in various game situations, notably an advancing Skitarii force that has to defend itself in combat with the Mindscanner Probes or the much needed melee and defensive improvements offered by Cognis Manipulators.
Best Uses - It is true that each unit in the game ultimately has one or a handful of tasks they are best suited for in a general sense; these are the most efficient uses of this unit based on their capabilities.
Given the long range of their weapons, the ability to move undeterred through all terrain types, their impressive durability and near inability to defend themselves in an assault hereby wasting the considerable shooting capabilities they offer, Onagers are best suited to roving around your backfield and deployment zone to safely devastate enemy positions from afar. Similar equipment choices and the unified target priority offered by that method of selection tend to be the strongest uses of Onagers given the sizable disparity of effectiveness against varying targets between each weapon, while safety in numbers is a motto that the unit is uniquely suited to following. I do recommend that nearly every Onager Dunecrawler squadron feature at least one Icarus Array regardless of other weapon selections due to the fact that Skitarii forces don't exactly have stellar anti-aircraft firepower elsewhere in the codex, though this obviously may be unnecessary if such threats aren't apparent in your gaming community. Position each Onager a safe distance apart up to the maximum allowance of unit coherency so as to minimize the effects of retaliation from high powered blast-type attacks, and be sure to move away from oncoming threats with their ability to fully ignore intervening terrain while re-positioning. Be mindful of using one or two infantry squads to cover the Onagers and deal with lesser close-ranged threats to them that don't necessarily warrant the full attention of the heavy guns, a tactic encouraged by the Broad Spectrum Data-Tethers each Skitarii vehicle is fitted with.
Thank you for reading this article, I hope you enjoyed it! Onagers are easily the most simplistic and direct Skitarii unit in terms of approach and usage in a standard war-game, providing heavy fire support from great distances as befits their name-sake - of course, they do possess the handy trait of being fairly mobile as well. I think they are a splendid addition to any Skitarii force as is justified by how often they pop up in small detachments allied to other Forces of the Imperium, though I am still curious to read your thoughts on them and how they have performed in your games. Thanks again!
To break with ritual is to break with faith.
- The Warnings, Tenets of the Adeptus Mechanicus