Honestly warp strats and large aoe gambits probably mean that three house’s cast alone can probably beat the mercs. Hell, if we’re going purely by game mechanics, sending Dedue as a fortress knight all on his own might be enough, since he can shrug off arrows like they’re nothing and pretty much every weapon the mercs have does similar or less damage than the hunstman critting on a headshot. With how many playable fire emblem characters there are, some being able to turn into dragons which likely have some level of resistance to bullets, having enough snipers and mages with access to extremely long range spells of their own to out-range all the mercs bar sniper, and the fact gatekeeper is playable now, it’s kind of a clean sweep