My favorite instance of cheesing a difficult boss fight was in Dynasty Warriors Gundam Reborn. Final battle was like 1 vs. 3 against max level Muso Gundams 1, 2 and 3. Knew there was no way in hell I was going to be able to pull that off through legitimate skill.
So I slapped some upgrades on the Deathscythe Hell that gave me effectively nearly unlimited cloaking and also let me automatically charge up my special attacks without having to do normal attacks, and just wore all three bosses down with hit and run tactics. Hide in stealth, unload all my special once it's fully charged, then run and hide in stealth again. Turns out the AI was completely helpless against attacks it couldn't see coming.