You mace smashes into the creature's arm, crushing bone. However, the creature flexes the limb and you can see most of the damage is already restored.
You sword slams into the torso solidy, but the wounds quickly close leaving only cuts that barely break the skin.
Your arrows strike the creature's hide, but are almost immediately pushed back out. The remaining wounds are barely doing more than scratching the surface.
I've noticed that if my players feel they are in a pinch, they might slip into meta-gaming a bit, and were I to use these kinds of descriptions, I'd need to wary that my players won't mix damage reduction with regeneration. Of course, there's nothing wrong with them making a few good blows trying to overcome the regeneration to realise that it's actually damage reduction.
It's good fluff, the sort I like, but the most meta-gaming gamists out there might get offended that they've wasted precious seconds of damage-dealing and blame you for it, but don't worry, they would deserve it. I find comments the likes of "My character would've seen through that. He would've known what to do from the start just by looking at the opponent."
Guess what? You don't even need to show up anymore, your perfect character already saved the multiverse seven times between the sessions so I've run out of adventures.
Uh, I might have slipped into ranting and venting a bit, sorry. But really, from your fluff I would say you'll have no trouble.
PS: I also like to sometimes describe how the attacks affected (or didn't affect) the monster mentally:
You arrow hits the demon squarely in the chest. His torso tilts back a little, but his speed isn't hampered a bit and he still stares you in the eyes, determined to rip you apart, and keeps his stride as he is getting closer to you.