If you are not using trance it is a lot harder. But the general idea is to maximize your melee damage.

Best way to do that is to remove their parry and shield block. Which is why kot is a good idea.

If you have saved gear and endure it should take them a few attempts to land serious maldictions. A full save wither will cost them 125 mana to drop you 5 str. Depending on your race plague might be something you just eat but saves can force multiple attempts in weaken/famish/blind and drop damage of demonfire.

Max your damage by hitting a vuln, nuking their defenses, and throwing lag. Minimize their offense by having divine saves, trance, and lagging them.

Two equally skilled players should see a lot of draws as there is generally plenty of time to retreat(on foot or by magic) because both sides need several rounds to land the kill. Keep calm gauge the progress of the battle and if it’s not tipping your way regroup and try again.

Note that duergar can’t see hidden in wilderness so you can still stalk them that way.