I always run a purely ashigaru force on Legendary, rarely bringing cannons 'for the lulz', and almost never using matchlocks in my army composition. Luckily, the AI can be beat with an equal-sized army on a regular basis, and with even less sometimes.
IIRC, in PvP you only get 40% of the attacker's army size for balance purposes: It is obviously very advantageous to defend. I'll mostly be talking about PvE, as that is my strong point, having played only a few field battles in the multiplayer portion of the game.
Tactics will vary depending on your army composition, and that of the enemy.
