See Challenge # 133.

Obviously magic doesn't usually futz around with removing parts of damage, but maybe this template is sufficiently obvious? Basically, it dies as normal, but is immune to double strike, combat-then-shock, and Pyrohemia.

Magic can cope with this just fine: see Ogre Enforcer! :)

Oh yes, good point. Although that was a while ago. Interesting how the flavour is so different!

Google suggests Callous Giant as a possibly-even-simpler implementation of a related effect, though the flavour might need to change to "hard shell" instead of "healing factor".

"Whenever ~ is dealt damage, remove all damage it from it" is probably the shortest template that works (since SBAs are checked before triggers happen), but most people wouldn't get that.