"Get a bucket. At a pinch a kitchen measuring jug will do but it must hold AT LEAST half a litre.
"Put all the dice in the bucket (or jug). All of them. No cheating. If you have less than ten dice, go buy some more before you try this.
"Roll. Count. Refill bucket, roll again, add to the score. Repeat once more — three bucketloads of dice. This is Cthulhu's current hit points.
"Cthulhu regenerates when damaged by 20% per [small unit of time] if still alive. If you somehow manage to kill Cthulhu, Cthulhu will still come back, but will take [score in hit points in minutes] to remanifest, fully recharged. NB: when Cthulhu returns, he will be angry."
Works for me - in the event they didn't manage to hurt Cthulhu at all, and he didn't actually notice they were trying because humans are just too small on his scale - the only human he noticed was Elvis, simply because he was using an amp loud enough for Cthulhu to hear him, and he quite liked the music, only trampling a couple of hundred people before he went off back to the sea.