I have more strategies,

1. Prince rennyn combined with strong archers like mitica headhunter, beatrix iron hand and unicorn mustang as tank. This combo will kill oshanus without giving them any chance to fight back because of high speed of those archers combined with one speed of prince rennyn.
2. Yodin zaku blasts combined with some good archers, they will also take out all enemy monsters with blasts easily.
3. Using opportunity monsters to take out all weak points.
4. Naga brute combined with delwyn dragonscale can also tank alot and kill oshanus with strong magic attacks.
   There are some more which I may add later