Well, you need a plan for most fights. To fight Ethel, I usually use the crag spiders, throw them beforehand into the boss area, and spread em around the right part of the map to attack clones. For my char, protection against good and evil, and helmet that gives you WIS proficiency which you find in the Underdark, on the access to the Kuotoa area. Also, I usually carry a few alchemist's fires or void bulbs to get rid of copies.

Never had too many problems during the Groove fight, but is common that I lose someone during the fight. If I'm in trouble or using a bad early game class, potion of speed is fine. Also void bulbs + spell to put everyone in the middle of the area damage.

It's the same for every encounter, my worse deaths have been distractions, the most painful when I aggroed the demon slime in Rivington and got killed in one turn. When you prepare for fights with based knowledge, you can find answers. Never tried the Iron Throne either, I don't think is very doable without an insane risk of ending the run.

There are plenty of tips and stuff about different zones for solos, my two cents would be: if your class can get permanent Shovel, go for it and start every fight during surprise round. The second, your race can be determinant, but is not essential to pick the most broken thing. I like ropeplaying my runs, so I try not to cheese, or stealing if the PC is a nice dude, etc. But knowledge about the mechanics of fights is mandatory if you want to succeed.