I actually did some of this a while back before I was on the forums. Back when GKC was new.
What you could do is have a counter with a property. Then, it’s subtracted every time the player’s attacked. Because each attack does the same amount of damage each time, you could have each one be a specific property and subtract the health property to the attack. Pretty simple. Critical hits and such could be easily done with randomizers.
For the health bar, you could do something with emojis, specifically 
and 
. You would need a and directly on top of each other in a straight line going sideways. 10 of them.
Let’s say a full health bar is 100 health, or . Each square signifies 10 hp.
Oh no! The player got punched! That punch did 10 damage. The squares (with a property checker) would change like so: . See how it changed out one with a ? Each is equal to 10 damage done.
Flying kick incoming! Let’s take a look at the health bar now. Using the same property checker method: . Wow, that’s a lot of damage. You can see how it goes now. 40 damage, 60 hp left.
If players have abilities as well (specifically healing abilities), then they could grant back.
Roundhouse kick! The health goes down: . This would symbolize that the player’s knocked out. You could be fancy and make it flash, too (deactivations and reactivations), if you wanted to. Then, the property checker would broadcast on a channel to end the game.