My story isnt quite exactly DnD but it was a board game that operated on very simmilar concepts (CONNAN)
Long story short the party (3 spelcasters, a Pirate, and Conan[a barbarian]) had been ambushed and had our equipment stolen from us. and our DM had given us the time limit to grab our stuff and get out (8 rounds/turns) as the "win" condition for the night.
The encounter proceded fairly normal for the gathering of equipment. by round/turn 4 the party was already against a wall as one of the quirks of the encounter was that the DM was able to summon more and more enemies as the encounter dragged on. the biggest downfall of the group at this point is that the 3 spellcasters required their magic token in order to cast their spells (Quirk of the game we were playing.)
This drags on until about round 6 when my character finally aquires his spellbook there is only one option left to him and it is to cast an AOE attack that would sacrifice himself but at the same time would wipe the board of all other remaining enemies. unfortunately for my character he would require a full round just to reach the needed (Ground zero) for his attack. so Coordinating with the other players my character reaches the area on the 7th round and is able to get all but one of the other player characters otuside ot the AOE for the attack. my character procededs to look at the other PC and gives him a thumbs up and says something to the effect of "Trust me this is going to work" and casts his spell.
To translate into a DnD equivelent the character was able to roll. it would be simmiler to rolling 2 Nat 20 for a double Crit and then rolling another Nat 20 for Damage.
So in a nutshell my character, the ther pc and the 5 other remaining baddies were engoulfed in a flash of light and the entire area that they were in vaporized instantly.
Unfortunaty for the rest of the party they were unable to get the last two chests of equipment and get out before teh 8th round and thus the party lost the battle but we went out with a bang…