Encounter: Dormitories (Chapter 8)
Select the Chapter 8 Dormitories encounter from the main menu. Any difficulty from explorer to hard is fine for this. The timer starts when you do the stealth kill. Remember you must do kills in a certain order: stealth, melee, headshot, explosive.
As soon as the encounter starts, rush to the enemy on the left wall as quickly as you can. Do a stealth kill on him, then roll away and do a melee kill on the other enemy on the wall. If you're fast enough and roll away he won't see you. If you're too slow and don't roll away you might get spotted and Sam will kill the enemy.
After the melee kill you'll want to jump down into the large enemy group. Use the AK-47 to go for a quick headshot and immediately do a quick throw with dynamite (press R1 for a short moment, don't hold it down to aim!). The quick throw isn't very accurate and sometimes misses. But the dynamite has a huge blast radius so it should be fine.
It may take a few tries to get the timing down. If you didn't get the trophy simply restart the encounter to try again.
If you have too much trouble with it you may even activate the slow motion cheat in the bonus menu (unlocked after beating game on any difficulty). The game counts time differently in slow motion.