OHKOing Escavalier is a near impossibility, creating a situation for the opponent once it's in to decide between the risk of staying in and being KOed by Megahorn or its appropriate coverage move or predicting incorrectly and losing a different Pokemon (or taking a ton of damage). Pursuit destroys the likes of Uxie, Roselia, Cryogonal, and most Cinccino, which have almost no way of harming Escavalier and face a lose-lose situation in either staying in or switching out. Iron Head is Escavalier's "safe move," providing a reliable STAB move that can also do more damage to those that resist Megahorn, such as Aggron and Golurk. The last moveslot is dedicated to Sleep Talk, as Escavalier makes one of the best switch-ins to nearly every sleep inducer in the tier, including Jynx, Lilligant, Amoonguss, and Tangrowth. Hidden Power Fire is a constant threat from these sleep inducers, but the threat of a Choice Band-boosted Megahorn once Escavalier's on the field is usually enough to scare them out, preventing any further setup, and even if they decide to stay in, an item change to Occa Berry can make Escavalier a dedicated counter to these threats.
Speaking of item, Choice Band is typically the best option for Escavalier. The sheer power of Escavalier's Choice Band-boosted Megahorn is what makes it the absolute threat that it is, and is one of the most powerful off-hand attacks available in the tier. However, because of the popularity of the Choice Band set and the inability of the opponent to risk the chance of not playing to the threat it brings, Escavalier has the advantage of bluffing pretty effectively with either an Occa or Chesto Berry. Occa Berry allows Escavalier to beat a handful of Pokemon it normally wouldn't be able to go one-on-one with due to the threat of Hidden Power Fire, particularly from the likes of Grass-types like Lilligant and Amoonguss and Psychic-types like Mesprit. Occa Berry even allows Escavalier to comfortably tank a Fire Blast from Absol, Aerodactyl, and defensive Slowking, which all typically carry the move almost solely for Escavalier. This item bluff is particularly effective when using Escavalier to Pursuit trap a Pokemon such as Uxie and the opponent switches in a Pokemon such as Absol to set up on the assumed Choice-locked Pursuit, only to be met with a KO from Megahorn.
Chesto Berry allows Escavalier to run Rest in the last slot over Sleep Talk, letting it either absorb a sleep attack once or use Rest to give itself a new lease on life and possibly tank and KO not one, but two of the opponent's Pokemon. This is particularly effective when Escavalier is taking on a setup sweeper like Hidden Power Rock Lilligant, which will typically use Quiver Dance as Escavalier switches in and then attempt to put it to sleep with Sleep Powder, only to have Escavalier wake up immediately and smash it with Megahorn. ChestoRest is fantastic against slow defensive teams too, where Escavalier will typically be able to out-tank at least one Pokemon, survive the hit of the incoming Pokemon and use Rest, and continue its relentless pummeling of the opposition.
With a Pokemon as slow as Escavalier, your initial reaction would probably be to ignore its Speed altogether and just focus on Attack and bulk. However, in RU, Escavalier resides in a very interesting Speed tier, meaning even at its snail's pace, speed is still the name of the game. The Speed EVs given on the above set allow it to outspeed minimum Speed Spiritomb, as well as Slowking and Amoonguss. Dropping down to 84 Speed EVs is doable to still outspeed minimum Speed Slowking and Amoonguss, and Escavalier gains a bit of bulk in the process. If you don't care about outspeeding, or figure that those defensive threats will be Speed creeping ahead of Escavalier anyway, max HP can be used instead to take hits more reliably. In particular, Sceptile and Accelgor have a much lower chance of 2HKOing Escavalier with Focus Blast, and Druddigon's Outrage will only 4HKO max HP Escavalier (assuming no entry hazards).
Escavalier's movepool is hilariously tiny, but Return is a definite alternative option to place in the last slot or over even Iron Head to nail Fire-types like Moltres and Emboar really hard. If you're choosing to not run Choice Band, Protect is a great option to run in the last slot to scout for Hidden Power Fire or Fire Blast and plan accordingly. While Knock Off is pretty weak, it's still a very helpful option to remove Leftovers and Choice Scarf from the opponent, particularly considering Emboar is one of the most common switch-ins and is significantly less threatening without a Choice Scarf boosting its Speed. Don't bother with anything else, honestly; Escavalier is very one-dimensional and that's fine, even if your opponent knows exactly what's going to happen, Escavalier will accomplish its goal.
As for teammates, Escavalier has great synergy with basically every other top-tier threat. Slowking in particular has amazing defensive and offensive synergy with Escavalier, and it can even set up Trick Room to initiate a one-two sweep by the pair as almost nothing else with outspeed them in Trick Room, especially if they're both running 0 Speed IVs. Alomomola has great defensive synergy with Escavalier as well, taking on the likes of Emboar and Steelix like it was getting paid, and provides Wish support for the otherwise-recoverless Escavalier. Escavalier works great in the middle of Regenerator and Fire / Water / Grass cores because it covers the attacking types that they normally miss out on. Escavalier provides the core's Druddigon check, as well as countering Sceptile and Dark-types and Pursuit trapping Psychic- and Ghost-types.
Due to most of Escavalier's checks being grounded and all the switches it forces, Escavalier works very well on Spikes-stacking teams. Spikes wear down the defensive counters to Escavalier that don't have reliable recovery very quickly, with Poliwrath and Steelix being prime examples. Escavalier even traps and eliminates Cryogonal for the team and can force Kabutops out. Finally, in general, Pokemon that love having Ghost- and Psychic-types removed from the battle, such as Hitmonlee and other Fighting-types, love having Escavalier as a partner. Offensive Grass-types, such as Lilligant and Rotom-C, also enjoy Escavalier's ability to either Pursuit trap or eliminate via Megahorn other Grass-types such as Amoonguss and Roselia.