Fun fun, I love seeing a good fight

. I need to ask a couple of questions first. Do you have any class, generic, category, or stat points left? It would be a great help if you did. Which is worse, its defense or armour? How often do you hit it? How much damage do you do when you do hit it? How many turns can you fight it before you need to run away (or go up the stairs)?
Okay, now a few suggestions. When you think you have a workable strategy--but not before!--I would use your speed rune whenever possible. Also, I would use Daneth's Neckguard as your amulet when it's not invisible. The enemy apparently has a rune of invisibility, which will eventually run out of charges. Don't seriously fight it until it no longer becomes invisible. To increase your damage output, I might replace Nightsong with your other gladiator's stralite ring. It might be worth trying to fight it with your slime-covered stralite waraxe of ruin. The slime will slow it down whenever you hit him. You could also try fighting with your quick stralite waraxe of ruin--the extra accuracy could be a big help. And, finally, I would suggest you use the "Precise Strikes" talent--the extra accuracy here should be worth the loss of speed.
Good luck!