"to cheese" a boss, a strong enemy in a videogame, usually means to exploit a certain mechanic or glitch in the game to make the fight substantially easier. In my example, you'd basically have the boss fall off the map for the victory.
Soulslikes are notorious for being very difficult games, so cheesing a boss could also mean that you'd use a bunch of consumable items, usually bombs and the like, to deal a lot of damage from a safe distance instead of engaging in melee combat.
Hope this makes sense :)