Twintania is a decently designed encounter, to use an example in FFXIV:

Multiple phases, each with their own challenges.
Tank and healer checks on Death Sentences and Snakes
DPS checks on Conflags and Dreadknights
Movement mechanics for all: Dive Bombs, Twisters and Hatches
Paying attention to debuffs: Conflags, Fireballs, Liquid Hells
Knowing timers for stuff: Dive bombs round 2, death sentence cooldown rotations, twisters, etc.

Plenty going on. In a well designed raid this type of boss would make an excellent early-mid tier encounter. Everyone has to know their role and positioning and nobody can screw up. Deaths can be recovered from, depending on who and when.

More like this would be a good start.