i got a long, long, list.
in DBZ: Budokai Tenkaichi there is a battle where u must survive 60 seconds as goku against gorilla vegeta. HE KILLS YOU ON 5 FREAKIN HITS.
Typhon in Titan Quest. he shoots meteors into the sky (he channels, and its only safe time to strike. if ur stunned by his lightning bolts, ur fucked. if u simply touch em, ur fucked. if ur stun UNDER the meteors, your barbecue.
Cerberus in Titan Quest and his cursed floor cracks. theres cracks in 1/3 of his room and when he growls, fires are started in em. if u graze em, ur crispy barbecue.
Ormenos in Titan Quest. its not instant kill, but his cave-ins damage a pain in the ass lot of hp. and after he has called the rocks down, u cant walk over them AND he can throw with them for 1/3 of your health.
Havent played it, but Sephiroth in KH2 is crazy. i think his "descend heartless angel" is all too powerful. all ur mana is gone and u is thrown to 1 hp.
The Sorceress in spyro 3. her lightning ball rain is fucking crazy. hits 1/2 of the room. ur ALWAYS hit once if ur not a spyro freak. (i only played spyro 3).
Aktaios in Titan Quest. about every 20% of his hp, he makes sunbeams 5 places around the room. if u step in those, ur dead on 2 secs.
Charan in Titan Quest: Immortal Throne. he makes some geysers of water around himself now and then, and if they touch you, your fucked. also, when u´ve killed him first time, he transforms and all his attacks damages about ½ ur hp. its fucking crazy. EACH AND EVERY SINGLE SPELL AND ATTACK.
Megalesios in Titan Quest. after fighting through the minoan Labyrinth and Defeating the original meteor, ur faced with this telkine. he is not that bad, his attacks kill u in 5 hits (thats really low in TQ). but he summons some demons called Limos´ that u have faced in the labyrinth and in greeke and on crete too. but instead of them getting hp, he gains it. AND THE HEALING IS ENOUGH TO MAX HIS HP ON 6 SECONDS.