Aside from trips...
including the backstab, you hit him 4 times:
>>> ANNIHILATES <<<
MANGLES
MANGLES
EVISCERATES

He hit you four times:
EVISCERATES
DISMEMBERS
DISMEMBERS
EVISCERATES

All the rest of the damage came from 8 backfists, mostly DISMEMBERS.

So, maybe reducing his dex if you can so he doesn't dodge as well. As others have said, vuln mace, but in this case, I don't think even that would have helped.

If you follow all this advise, I still don't think you'll come out on top because next time the assassin can trance and kote or kans instead of tripping if he feels he's in any danger. Though not as bad as other thieves, thugs are still weak in melee compared to other melee classes and I think that assassins are the best. That's why so many thieves either stay at L42 or hero and run around grief robbing people.