For half of the answer, visit:
http://social.biowar...8/index/6681852
It is a list of elemental resistance and immunity on Nightmare mode. It is not what you are looking for, but it is half of the answer. Say, rage demon is a fire elemental, it is naturally immune to fire attack, and it does fire attack. In other mode, nobody is immune to anything, just have higher resistance.
First, your mabari is immune to Rage Demon attack so he makes a perfect tank, only if you can tell him who to attack. This can easily conclude that Rage Demon does fire elemental attack. And I do not think it differs whether it is elemental or magic damage, they should be all the same.
Second, I have tested that my 33dps weapon that does Fire damage did a whole lot more damage to golem than those much higher physical weapon dps (around 42-48). This is is because fire weapon does not check target's armor. There should be a few weapons in the game that are not physical damage. This should also conclude that a Rage Demon would do nothing to you if your fire resistance is very high (hypothetically speaking). Aegis armor would help greatly in this case.
Note: just by looking at the list scanning for immunity, it does not 100% tell you what type of attack they are using. Spiders spit poison which does Nature damage, but their melee attacks do physical damage, I am quite sure. Rage demon is pure fire, as I watched my mabari taking 0 damage from it (even on Nightmare). Qunari have immunity to Lightning, but their attacks are strictly physical.
It is quite obvious to tell in most cases. But some, like Revenant, I still haven't decided whether it does physical or frost damage. It sure casts frost spell, but it hits you with a sword. Common sense tells me it does physical damage on auto attack, but ... my common sense has nothing to do with how Bioware designed the game. All human enemies do physical damage regardless of what immunity they have (mages are exception of course). Abominations do physical damage I think. Shades .... not too sure whether physical or spirit, I haven't yet had a chance to find out.
Hope I helped a little.