Etherian Goblin

Every wave of the Old One's Army includes these, on every tier. It is the bulk of the event, the body count your sentries are meant to be grinding through while you deal with whatever flies or throws. Its equipment and toughness scale with how far into the game you are, so the goblin you meet on a first Tier 1 attempt is not the one that arrives on Tier 3.

You cannot wall these out. Lose line to it, or stand too far off, and it goes ethereal and walks through your blocks until it can reach you or the Eternia Crystal again. Lava does nothing and teleporters ignore it, which removes the usual arena tricks. Build for damage instead: clear lanes, sentries covering both approaches, room to back off while they close.

Up close it simply walks in and swings, so the danger is being surrounded rather than any single hit landing. Keep moving along the lane and never let a group pin you against the crystal. The banner wants twenty times a normal enemy's kill count, so it comes out of repeated full events rather than one good night.