After a rather fortunate bones find, including such loot as a wand of clonign, two scrolls o charging, 17 scrolls of change material and other pretty useful stuff, I proceeded to whack Elpuri for the second time. Now I knew I wouldn't probaly make it all the way to Oree to get the shirt, I headed straight back to Petrus. After reaching him I saved my game and took a backup copy so I could test some viable knight victory strategies. After some seven tries, I do believe I've found the most effective strategy.
And for those members of the community who were already fetching their pitchforks upon hearing about a backup copy of a save game, I'm not pretending I actually made a knight victory. I merely used the state of my Elpuri defeating character to have a realistic character about to head against Oree. I've tried it in wizmode several times but usually it ends up being not even close to what it is in reality, thus leading to improbable situations.
So, here's the ultimate Oree Slaying For Dummies guide to those unfortunate enough, me included, who are yet to slay him in a legitimate game. What you need is obviously a character powerful enough to kill Elpuri. If you are that strong, you should probably be strong enough to kill Oree as well. You also need telecontrol, a bunch of scrolls of detect material and a reliable way to teleport. The main idea is not even trying to fight your way through the levels. That more often than not ends in a disaster. In stead, upon entering a level use a scroll of detect material and search for granite. This should reveal to you the location of the stairs down. Teleport as close as you can to the stairs and head down. Don't stop to fight, unless you have to. Upon reaching the level with vast amounts of mages and the mystic dark frogs (GC 12 I believe), search for gold to locate the portal. Teleport immediately to it and proceed. This level is the biggest killer. More often than not, you'll die almost instantly due to your equipment teleporting away and magical lightning killing you. After going through the portal, you should find yourself at Oree's lair. Search for golden eagle feather to find it and teleport next to him. And, well, kill him. Take the shirt and leave as soon as possible.
Using this strategy, the only real problem you face is killing Oree. You'll spend almost no time at all in the levels beyond Dark Level. Wands of teleportation are faster to use than scrolls, so they should be your preferred method of teleportation. Killing Oree should not be a problem for a character cabable of killing Elpuri, but then again given how this is IVAN, your attempt may end up just as easily in Oree annihilating your groin.
And as a last bit of advice for those uncertain of what equipment to use for the endgame, you'll need high fire resistance, high electricity resistance and telecontrol. ESP is highly useful. Infravision not so much, but if you lack ESP it is a decent way to compensate for it. Being hasted for the final fight helps, this can be achieved through use of wands. So, if there are still those who haven't achieved the knight victory frequenting these forums, I hope this helps. I hope it helps me the next time I slay Elpuri and decide to proceed further.