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Boss HP can be find here
In terms of resistance, bosses don't have a specific resistance (except in the case of to-hit debuffs) and depends on which boss you are fighting. However when fighting above your level your effects will be subjected to the purple patch(click).
That means that you are 65% effective against +3 and 48% effective against +4s which you might think about as a resistance. I can also give you the details of to-hit scaling for enemies if you're interested.
And because I appear to have that data in my guide so it was easy to find:
Bosses have an accuracy modifier of 1.3 and resist to-hit debuffs by 20%.
Accuracy Modifiers based on level are here and for +3 is an additional modifier of 1.3 and for +4 is an additional modifier of 1.4.
Draggynn on Virtue: lvl 50 Storm/Psi, 1389 badges
Draggynn's Guide to Storm Summoning(Gale-Tornado, updated 6/25/2011)
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To find the resistance of a specific boss vs. various damage types, check Culex's Enemy Resistances - Damage and Mez thread.
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I am trying to calculate a build/team and I need to know a few things.
My team will likely be fighting lvl 53/54s. I need to find out the resistances and HP of lvl 53/54 Bosses.
My other question is, does HP vary from a lvl 50 boss to a lvl 54 boss?