Read the log!

Just a small reminder to read the logs when fighting foes that won't die. Took me a while to realise that I healed the radiant horror when attacking him with my fire-gauntlets :D

A little late in my reply here, but...

You can (and should, especially in higher levels or against rares/uniques/bosses) just hover your pointer over the monster to see a short description, including its resistances. To learn its affinities as well, right click and pick Inspect Creature. It'll bring up the monster's Character Sheet, like the one you get for yourself, including all the data you need: numbers, weaknesses, strengths, attacks, effects, spells, powers, runes, inscriptions, etc as you cycle through the pages. It can be a lifesaver. Appropriately enough, I really got serious about doing this sort of inspection while focused on battling a radiant horror surrounded by fire-vulnerable monsters. I use fire spells, knowing that the radiant horror was immune but not knowing he had affinity. I figured I could afford to blast away with fire every couple of turns to clear away the others so I could focus entirely on him, not realizing I was healing him back up in the process until all the monsters except him were gone and he was back at full health. I'm pretty sure I died a few rounds later because I had to start killing him all over again while badly drained of resources.