Golem

Golem does not wander in on his own. You carry a Lihzahrd Power Cell to the altar in the last chamber of the jungle temple and activate it, and he assembles directly above that altar. Time of day is irrelevant, but the world has to be in Hardmode with Plantera already dead, so the temple key and the boss behind it sit behind the same gate. Each attempt eats one cell. There is one catch built into the altar itself: the bricks holding it in place cannot be actuated until you have beaten him once, so the first fight happens exactly where the game put it.

He is four separate targets. The body cannot be hurt at all while the head is attached, so the entire opening goes into the head. The two fists are optional and that choice costs you either way. Killing a fist removes a stream of damage but makes the body jump around more often, and taking both out leaves you chasing a very mobile target. If your damage is high enough to burn the head down quickly, leave the fists alone.

He jumps, over and over, trying to come down on top of you. If solid blocks break the line between you and his body, the jump becomes a leap that passes straight through terrain, so hiding behind a wall relocates him onto your head. The fists fire on chains with a short reach and glow orange for a moment first, and that glow is your cue to move; they only launch when you are between them or on the same side as one of them, so staying wide of the body cuts them out of the fight. The head spits ricocheting fireballs that set you alight. Once the head drops past half its health it opens up with eye beams that travel through blocks, and from that point everything it throws comes faster the more you hurt it.

This is the mistake that turns a manageable fight into a loss. Putting blocks between yourself and the head roughly doubles how often the beams arrive, and once the head has detached, an obstructed line also makes them hit considerably harder and travel much faster. There is no safe wall in this fight. Alignment matters on top of that: the head fires from one eye while you are off to a side of it, and from both at once when you are level with it, so keep yourself above or below its height rather than square on.

Killing the head does not remove it. It pops off, becomes untouchable, and floats above you through the terrain, firing both beams from then on. The body is the target now, and it speeds up as it loses health while any surviving fist swings more often. When the body finally goes down the free head dies with it and the fight ends.

Two odd side effects come with this phase. The body takes damage from lava, which almost no boss does, so a shallow pool under the arena finishes the second half faster. And the map icon tracks the head while the despawn check watches the body, so running from the head while the body falls behind is how people accidentally lose him.

Fight him in the underground jungle. Outside it he enrages, jumping quicker and, once the head is past halfway, throwing beams faster as well, and he settles down again the moment he is back. That is the argument against moving the altar with a Picksaw after your first kill: the convenience buys you a permanently angrier boss unless you rebuild inside the jungle. The temple floor is uneven enough that he sometimes wedges into the terrain around the altar on spawn and can only shoot, never jump, which is a far easier fight, and you can set that up deliberately with a few blocks. Two smaller notes: everything he throws comes faster with more players on the server, and he is one of the rare bosses that low gravity in space actually affects.

This is the gate on the last third of the game. Cultists start gathering outside the dungeon, and killing them summons the Lunatic Cultist, whose defeat opens the lunar events, provided Skeletron has already been dealt with so the old man is not occupying that spot. Martian probes begin drifting across the sky, and letting one escape triggers the martian invasion. Summoning the Old One's Army from here on runs its hardest tier with Betsy at the end.

The quieter changes are worth collecting too. The Steampunker adds Steampunk Wings to her stock, plus a lihzahrd furnace if she lives in a jungle, the Party Girl starts selling the Celebration, and lihzahrd furniture becomes reversible in shimmer.