Another unorthodox alliance that I think can actually be competitive is using a Harlequin Masque in conjunction with an Armoured Battle Group allied detachment. Consider the following:
-Pros — Highly mobile, powerful psychic options, exceptional in close combat and solid short-range firepower
-Cons — Expensive, fragile, lacking effective long-range shooting, lacking anti-flyer options
ARMOURED BATTLE GROUP
-Pros — Highly durable, multiple effective long/mid-ranged shooting options, Objective Secured AV14 units, effective AA shooting
-Cons — Expensive, slow, highly vulnerable in close combat, no Psychic options
As you can see, in may ways these two armies are mirror opposites of one another: one is close-combat focused “glass cannon” force and the other is firepower-based “anvil” type faction.
Because of these differences, I think the two forces compliment each other quite well. Establish a firebase with the Armoured Battle Group to begin impacting the enemy turn 1 and tuck the Harlequin units in behind the wall of AV14 and AV12.
If the enemy tries to out-shoot you, slowly advance your Leman Russes/sheltered Harlequins while pouring on the firepower of the Armoured Company. Then, once you are within range (say turn 2, 3, or 4), have the Harlequins dart out from behind their tank shield and use their 6″ move + D6″ run + 2D6″ charge (all with Fleet) to get into the enemy gun line and chop it up. This can be highly effective against Tau, Astra Militarum, shooty Necrons, Skitarii, Cult Mechanicus, shooty Astartes (of all variants), and even some versions of Eldar (i.e. footdar, gunline Eldar, etc.).
Alternately, if your opponent moves in close to either try and assault your tanks and their vulnerable AV10/11 rear armor (i.e. Tyranids, Orks, Khorne Daemonkin, Grey Knights, Daemons, assaulty Necrons, etc.) and/or use close-ranged anti-tank shooting against them (i.e. Melta Vets, Fire Dragons, Fusion Blaster Crises Teams, Tyrranocyte Zoanthropes, Drop Pod Marines, D-Scythe Wraithguard, CSM Combi-Melta Terminators, etc.), then your Harlequins can “bubble wrap” the vulnerable Leman Russes or, better yet, pre-emptively charge your opponents, thus keeping your tanks safe/still shooting and also tearing them up with your highly potent Harlequin close combat abilities.
Either way, with skillful integration of these two factions, you place your opponent in a dilemma and can combine the best attributes of both factions into you battle plan.
In support of this Harlequin/Armoured Company Battle Group concept, here are what I consider some of the best units to take:
ARMOURED BATTLE GROUP
1 – Command Tank (HQ)/Commissar Tank (Elite) — Both have BS4 and access to special Tank Ammo, the best of which is the Monster Hunter ammo (S8 AP2, Small Blast, Instant Death) on Vindicators and Infernus ammo (S8 AP3, Ignore Cover) on normal Leman Russ Battle Tanks. The Command Tank can also issue orders, the best of which enables friendly tanks to target an enemy and force them to re-roll successful cover saves, and they both can take upgrades like Aritificer Hull for +1HP. Overall, these make for strong anti-tank and/or anti-infantry shooting to lead your force, as well as more AV14 saturation.
2 – Siege Tank Squadron (Troops) — 1-3 Demolishers, Executioners, Punishers, and/or Thunderer Siege Tanks (basically a turretless, cheaper Demolisher) in a squadron. With tremendous firepower (i.e. S10 AP2 large blasts or 3 x S7 AP2 small blasts or 20 x S5 Ap- shooting), AV14 in the front and AV11 in the rear, plus Objective Secured for being a Troops option, this squadron type can relentlessly advance, protecting harlequins behind it, and pound the enemy while doing so.
3 – Vendetta Squadron (Fast Attack) — 1-3 Vendettas provide excellent anti-tank, anti-flyer, and anti-Monstrous Creature shooting at long range to round out your Harlequin/Armoured Battle Group task force. Not cheap, but can be your “ace in the hole” unit in many games.
4 – Collossus Bombard Battery (Heavy Support) — 1-3 Collosus Mortars that eahc put out a S6 AP3, Large Blast, Barrage, IGNORE COVER shot per turn. Obviously, this is death to any MEQs, Necrons, Eldar Jetbikes, Wraithguard, Nob Bikerz, Tau Crises Suits, or basically anything without either a 2+ save or a transport to ride in. If you are facing an enemy “gunline” force, then this is one of your best compliments to the tank/harlequin foundation of the army and, even if you are facing an assault-oriented horde opponent (like Ork green tide or Black Templar black tide army), then these mortars can even scale. Finally, they even have utility against medium enemy armor, since they are S6 Ordnance and Barrage (meaning they hit side armor), which can be quite potent against things like a Space Marine Battle Company spamming free Rhinos/Razorbacks.
1 – Solitaire (Elite) — Normally considered somewhat mediocre, in this list I think he shines. As a single model that can easily hide out of LOS behind advancing Leman Russ platforms, he cand wait for the opportune moment, then dash out 12″ (or even use his Blitz ability, if appropriate), then run another D6″, then charge another 2D6″ to single-handedly tie up (or even destroy) an enemy squad. In this he maximizes his excellent close-combat abilities and also serves as an effective “economy of force” unit for you army (i.e. if he ties up a squad of TWC about to multi-charge your expensive Leman Russ Executioner squadron and rip it to shreds, then he has basically made his points back already). Don’t forget he has I10 (so can basically auto-win Sweeping Advance rolls) and Hit and Run, so he can charge, beat up a unit, then H&R out to allow him to charge another unit next turn.
2 – Shadowseer (Elite) — Another great froce multiplier unit, with each Harlequin Troupe probably needing one of these psykers attached to it for Veil of Tears if nothing else. Combining an AV14 armor wall to block LOS with Veil of Tears (for any enemy units that are fast enough to zoom up the flank and actually get a bead on the Harlequins) on each Troup and you can all but ensure that the enemy will not harm them until you are within charge range for your deadly clowns (which is where you want them, of course 😉 ). Also, the Shadowseer can “defang” critical enemy units like FMCs, Centurions, Superheavies, etc. with powers like Fog of Dreams (basically “reverse Invisibility” cast as a malidiction on an enemy unit and forcing it to Snap Shoot and only hit on 6+s in combat for a turn) or, alternately, combine the Mask of Secrets Relic (-2 Ld on nearby enemies) with Psychic Shriek power to inflict signficant witchfire damage on even very tough enemy units.
3 – Harlequin Trooupe (Troops) — Your “bread and butter” close combat unit. If take on foot in squads of 8-10 with the right close combat upgrades, then sheltered behind friendly armor/with Veil of Tears until they can charge, this squad can hit well above its weight and provide exceptional close in protection for your slow, lumbering, vulnerable-to-assault tanks. Personally, I think the Harlequin’s Caress is the most versatile upgrade option, making them Troupe a threat to infantry, MCs, and vehicles alike and combining quite well with the supporting firepower of the Leman Russes and Collosus Bombards at range.
4 – Skyweaver Jetbikes (Fast Attack) — With all the advantages of Eldar Jetbikes (i.e. ridiculous mobility, making them tremendous for grabbing objectives), plus the option to either take ranged Haywire shooting or lots of ranged Shurikan Cannon shooting, plus the ability to become potent close combat units on the charge (i.e. 4 x S5 AP2 I6 attacks on the charge with Zeiphergalive upgrade and the option to Hit and Run out of combat to “rinse and repeat” this tactic), Skyweavers bring an element of hyper mobility and specialized shooting/close combat that really compliments Harlequins Troupes on foot and massed Imperial Tanks. Consider these guys your QRF (i.e. Quick Reaction Force) which can dart around the battlefield and do whatever role you require of them, be it grap objectives, assault an enemy shooting unit far from you DZ, or add specialized shooting to support the firepower of your Leman Russes and artillery.