Yeah, don't use the Really Big Sword. You'll be paying through the nose to do so. Secondly, if you encounter a high physical resist boss, Taurog is usually a horrible selection that will actually hurt your chances more than he will help. If you're on a multi-boss dungeon then picking Taurog early is a big gamble. I've actually written a few times that Taurog needs a rework because in the more difficult dungeons he is very often more of a penalty than a benefit to anyone who isn't playing berserker or monk (two classes that just scream for Taurog's boons).
I'm presuming your talking about double-doom, so there's really no way for me to give you specific advice against specific bosses. You want to start early when there's still tiles to exlpore, and you want to avoid using potions as much as possible. Specialist characters will usually have one "easy" boss that is weak to their particular strengths. Paladins and Monks have Super Meatman and The Ironman, Berserkers have Firstborn and Aequitas, and just about anyone can kill Tomithy in a heartbeat. Don't waste potions, attack early (probably level 6-7), use a mid-battle levelup to make it even easier, and you will have the game down to a 1-boss battle with all your resources intact. Once you've knocked out the easier of the two bosses, you can direct your full attention to the more difficult one. If you wait until you're level 9 and the entire dungeon is cleared to start fighting the first boss, you're in for a rude awakening. You want to start early and spread out the battle.
As for the 10-bosses in a row battle, that one basically revolves around creating the "ideal" level 10 hero. It's a very challenging match and only a handful of classes even have a remote shot at beating it (Thief, Warlord, Rogue, and Priest are the only ones I'd be inclined to attempt)