The Guardian

One of the three Act 1 bosses (alongside The Ghost Hexaghost and The Boss the Slime Boss), picked at random each run. The Guardian shifts between Offensive and Defensive Mode - it goes defensive after you deal enough damage and grows big Sharp Hide/thorns, so stop attacking while it's in Defensive Mode (you take the damage back) and unload while it's Offensive. You'll meet it just by playing Act 1; which of the three you get is down to the run's RNG.