Kovik the Splicer Priest in Destiny 1.

He was a boss at the end of a "strike" mission. You fight him in a circular pit with a couple of small hiding spots. He shoots a big laser at you the entire time, unless you stun him, and it is painful if you needed to move from cover-to-cover. Why? Well, twofold: first, he had the ability to teleport to your position, or a position in which he could flank you, and stomp/shoot you to near death. Secondly, he has a bet ogre who stomps around the battlefield and cannot be killed. He takes no damage, cannot be stunned... there is no way to stop him from hurting you with his big slam. At certain intervals, minions would spawn making your already limited cover useless. Not only that but the ogre would begin running at you. I don't mind being forced around the battlefield for a boss but Kovik's gun was a laser that had excellent target acquisition: it was difficult to outrun that. With minions, an unkillable ogre and little cover, it's just a pain in the arse.

This is not to mention the modifiers that sometimes the strike would give you. If there was one where a particular damage was active, the minions who drop from the sky into the pit would kill you in two bullets. Actually, they didn't drop down, they slowly floated to the surface before extensively tracking the players. If you didn't deal with them in that hectic environment, Kovik would punish you.

Similarly, there is a strike boss in Destiny 2 called Thaviks the Depraved. He is a boss that randomly teleports out of the battlefield. You have no chance to stop him. When he does appear, he is invisible and there are few indications of where he may reside. As soon as you find him and begin pumping some damage, he leaves again. It is a randomly selected strike that everyone leaves as they fly in because they wish not to deal with his bullcrap. This is outside Destiny 2 being a bad game altogether and the main storyline villain being so easy to destroy it was laughable and when the villain finally became interesting, the game Deus Ex Machina'd the players with a "character" nobody really cares about.

Destiny had some great bosses, like Oryx, Sepiks and Atheon, but damn when it was bad, they went really bad.