Two separate encounters.
Firstly, a cockatrice on Bitterblack Isle. I was completely unprepared, only brought three panacea and no secret softeners or anything of the sort that could cure petrification. Over the course of the battle it killed all three of my pawns, because I they were petrified and couldn't be cured. The last pawn, my healing mage, was still technically alive at the instant I finally dealt the finishing blow. She died immediately afterwards, though, and the cockatrice didn't drop anything that could have saved her anyway. I snuck back to the entrance, because I didn't have a liftstone or anything to teleport. I say snuck in only the faintest sense, because there's no real sneaking mechanic, and was spotted by goblins.
Encountered an elder ogre in some sewer-like tunnels, still on Bitterblack isle. This is a monster that has 10 health bars (1 bar + 9 dots). It is huge, it repeatedly picked up my pawns and chomped on their heads. The only attack I had equipped that had any real effect against it was the explosive ricochet shot from my bow. We fought it back and forth in a section of the sewers, and very, very slowly took out 4 dots of health. Half dead. Then by pure luck it charges through and past the party as we have our back to a hole in the wall, and it goes right through the hole! There's a narrow ledge on the other side where it lands, and sits there for a minute while I try to convince my pawns to pull back. Then, it jumps off into the water below and dies. Instant victory, 25k experience. Suddenly wish I'd thought to use one of my veterans' periapts so I'd get the double XP, but the effect wouldn't have lasted the entire battle and seeing it off a cliff was completely unexpected. In fact I thought this would be a repeat of those two separate times a cyclops bludgeoned me off cliffs to my death, when it ran out of the tunnels and to a cliff overhanging that chasm, so I lead the party back inside once rescuing the warrior.
Dragon's Dogma: Dark Arisen