[Scripting Question] Help with creating a scrip for a semi-complicated ability

I need help creating a script for a new ability for my fan game.

The ability is is called spray tag, and the way it works is after the pokemon (smeargle) uses a damaging move it applies a "mark" on the opposing pokemon and it stays active for two turns. After those two turns the mark pops and deals 20% extra damage to the marked pokemon depending on how much damage the user of spray tag has inflicted during the two prior turns (including the damage from the first move that applied the mark.)
I think this a really unique ability and I really want to see it work. I'm not very good with Ruby, which is why I could use any help I can get, and I would really appreciate getting help to make this happen.

I think you should look at how Bide works, because that's another effect that counts how much damage has been dealt over a number of turns and then does something with it.