If you think lagging before you know if you're going to win is the right answer, you're an idiot. Duergar in question was also spamming commands a bit which is a huge no-no in general, but especially against assassins who have big maledictions that can turn the fight immediately.
The assassin is a village zerker, he had resist up which gives him a bit more 'meat'. Standard dark-elf assassin in the low ranks is eating only 3-4 hits from a well-geared duergar mace spec.
And assassins have shitty hp gains compared to a warrior. A dark-elf assassin will have less hp than a dark-elf warrior. The classes ARE different. The duergar has a one round lag damage skill. Fucking use it. Spamming bash on a village zerker assassin doesn't prove anything except tactical shortcomings.