So unfair tricks you want to know, heh? Mmmm, so listen now. All your stratagies are fine, but here's the most brutal devastating trick I was ever been able to discover.
You need: THIEF/MAGE able to cast 7th level spell specialized 2* in large swords. Set backstab in config instead sneak attack. And you need nothing more, nothing less.
If you are alone, first summon some creatures. If you are not, just use your tank in your party.
Then...
Cast spells in this order: POWERINVISIBILITY (i think it is called IMPROVED INVISIBLITY), HASTE, MODENKAINEN'S SWORD and TENSOR'S TRANSFORMATION.
Then with your tank (or summoned creature) pass behind your enemies, so they turn back to your boosted mage and watch, what is real power of destruction. Even the strongest opponents like golems and giants falls after 2-3 hits in this killing frenzy. You can even drink potion of STR 20 or so before and look, as blows with damage 100 and more falls one after one in tremendous quickness, and how your opponents falls one after one. Thus, in one round can mage/thief deal MORE THAN 400 DAMAGE!
So and then speak about the best damager ever

)
Tip 1: If you have another mage, you can cast improved invisibility at the end at your boosted thief/mage, and so place him to optimal position in battlefield, without any enemy notice.
Tip 2: You can still cast several more spells or potions to boost your thief/mage even more. (THAC0, AC, PROTECTION...)
Important note: You must attack your enemies from the behind all the way, or no backstab damage will occur.
ENJOY.