There's no strategy. Select your healthiest monsters and whittle the heroes down enough that the boss can finish them off. Writing is typical millennial cringe. Leveling your boss monster grants largely inconsequential 5%/10%/20% bonuses, nothing exciting or game changing. Unlocking new monsters and traps is all RNG so just pick whichever is strongest since there's no synergy between monster types or abilities. Focus on hurting HP or Morale, whichever attacks do bigger numbers. It's all very boring.