Well I gave this a whirl, it works with the normal combat system, but I'm not sure how well it will work with yours, being as unique as it is. Anyways you will need to make three variables and use two switches for this...rather messy, but I could not find a simple work-around. The first thing you'll want to do, as I'm sure you know, is make sure that the skill calls a common event, have that event turn on a switch. Now, after that, you will want to set up your troop of that monster with these details on the first page.
I know it looks odd, but basically what this does is have the variables be triggered to start the count down and comparison.
Next you will want to make the second page to look like this.
This will check each round to see if the hp is 300 less than before, and if not it will start counting. Once it reaches 3 it will attack, otherwise it fizzles. Does this make sense? If it doesn't work or you have questions, let me know and I'll be glad to run by it again.
Also the only thing you are missing from that second screen shot is where it says Branch End again.
One other thing I noted, is that you don't really have a way to keep the monster from restarting everything by using the skill again, so if I were you, I'd only have the skill be used by an in battle event call, it makes more events but gives you more finite control of it. Also if you choose to do that, you won't need the actual skill or the common event, as you can just set the switch when he 'starts' the attack. Does that make sense? If not I can post a screen shot of what I mean.
Edit: Took another look at this, and it gives a free round before the count down actually begins for the player to deal free damage, you may account for that with the damage amount or by dropping the number of turns in the variable down by 1, and have it say it's first message when it starts the variables.