The Slime Boss is one of three possible bosses for Act 1.
He starts with 140 HP and will split into two medium slimes when his HP is reduced to half or less.
The two medium slimes will each start with however much HP the Slime Boss had when he split. They will split into two small slimes when their HP reaches half or less and the two small slimes will start with however much HP the medium slime had when it split.
If you manage to reduce any slimes HP to 0, they will be defeated instead of splitting.
The Slime Boss goes through a three turn pattern:
Turn 1) Goop Spray- shuffles three Slimed
cards into your discard pile.
Turn 2) Preparing- does nothing for the turn
Turn 3) Slam- deals 35 damage
You ideally want to deal 70+ damage by turn three (so he splits) to avoid his slam, but you also want to balance out how much damage you deal beyond 70 by maybe instead tanking the turn 3 slam since the resulting medium slimes will each have the Slime Boss' remaining HP as their starting HP. If you can only get him to, say, 70, it may be better to wait another cycle (tanking the slam as best you can) and get into position where you can get him down well below 70 to reduce the medium slimes' starting HP.