Idealistic Perfect Run Scenario:
-Annie sweeps Sidney, no damage taken
-Against Pheobe we switch out first turn to doge, he kills 2 mons and goes down
-Next we switch to Vileplume, she takes out 2 mons and goes down
-Finally we switch to Tentacruel and he takes out the final mon OR switch to m4 and he uses Surf. Rollout is just free damage for the opponent while it builds up power
-Annie sweeps Glacia, down to below 50% hp
-Annie defeats Shelgon, Drake switches to Altaria and she goes down
-We send out Graveler, he takes out Altaria and goes down to Kingdra
-We send out Tentacruel, he takes out Kingdra, then Flygon, then Salamence (this part I'm iffy about. From full health he can probably take out Kingdra if we use mostly HP Electric, then iirc can 1hko flygon with surf, and then I'm not sure if he can take out Salamence at this point, but Salamence takes a lot of hits to beat him so maybe)
-Tentacruel gets some damage off on Wailord, but faints
-M4 rolls to victory from 100% hp
This isn't actually that ridiculous to pull off aside from the switching to the right pokemon and using the right moves with Tentacruel. Even if he dies against Salamence but damages it enough M4 should be able to take it down in 1-2 turns taking minimal damage and still being healthy enough for Wallace, provided rollout doesn't miss.