|This is a player phase build designed to make Roy as offensively devastating as possible, especially with the proper horse buffs (specifically Hone Cavalry). Speed is the ideal boon here as it allows Roy to double more opponents. Resistance is the bane of choice as the only units that can counterattack Roy (outside of dragons and Close Counter mages) all deal physical damage. Obviously, we keep his legendary Blazing Durandal, as it increases his attack by 3 as well as has a built-in Heavy Blade 3. The assist is up to player preference: Reposition is usually recommended for melee units, but Draw Back is a viable option as well, especially given Roy's mobility as a cavalry unit. Keep Galeforce as his special; despite its high charge time, it's relatively easy to proc thanks to Blazing Durandal's built-in Heavy Blade. If Roy procs his weapon's passive while doubling a unit that can counterattack him, he will activate Galeforce on that same turn, and can charge off to vanquish another opponent.
For his A skill, Swift Sparrow 2 is selected. The speed it gives is very welcome, as it allows Roy to reach 42 speed upon initiation on his own, while the attack boost allows him to more reliably activate his weapon's Heavy Blade (55 attack, which is one more than a +Atk Ike). Life and Death 3 is an alternative choice since Swift Sparrow is difficult to come by. The reduction in bulk can be made up for with a Fortify Cavalry buff.
There are two solid options for his B skill. Desperation 3 allows Roy to be more offensively oppressive by allowing him to make an automatic follow up once his health is at or below 75%, and thanks to his good speed in conjunction with a speed boon and either Swift Sparrow or Life and Death, he will be doubling quite often. More often than not, the only units he won't be doubling are high-speed mages (like Nino, Celica, and Linde), Ryoma, and Lucina. He will have enough power to one shot the aforementioned mages as they are physically frail. If Ryoma or Lucina pose an issue, consider running Swordbreaker 3 instead. Most Ryomas run Vantage and most Lucinas run Desperation or Renewal, so Roy can reliably double these units and ORKO them as a result. (Keep in mind that since he is doubling a unit that can counterattack in this case, he will activate Galeforce and can take another turn.)
The C skill is usually based on what your team needs. Hone Cavalry and Fortify Cavalry are good choices for a horse emblem team, and are relatively easy to come by as they come from Gunter and Jagen respectively, two units that can be pulled as 3 stars and 4 stars.
The Speed +3 Seal is recommended so Roy has a better chance at doubling opposing units.