For the Queen of Corrosion boss fight, if you only have melee weapons on you, rushing at her can be suicidal; instead you can use your spikes to hold and push the harmful floating spiky debris. Find these spiky balls and push them towards the boss spawn; as the boss chases you it will damage itself on these. Make sure not to touch spiky balls with unprotected parts of yours, only with the points of your spikes.
As for the infamous Leech Lord or Cage Boss as I call it, here is the strategy I came up with: as I enter the room and boss spawn animation starts, I rush upwards, hide behind the boss as it spawns and eat away at his unprotected brains. Easiest boss ever once you know the trick.
