Adding Poison States that decrease HP or MP in real time in of battle

Greetings!
What I'd like to do is add a poison state, that decreases the players HP/MP in real time during battles/battle selection menu to promote pressure when selecting choices.
I managed to get this to work with parallel processing common events, but only outside of battle. I'd like to get it to work in battle.

I'd like the players to be able to:
- Visually see the player's HP/MP bars drain in real time while they select an action.
- Drain the actors HP/MP by a percentage

I've tried to work it without scripts, but I think I might need them after all. Of cause, if this can actually be done without scripts then that would be most welcome too ^^
Thank you all very much in advance :D

'Scripts' is where people who have written completed scripts they want to share with others can post them, it's not for asking for scripts.
[move]RGSS3 Script Requests[/move]

Oh, my apologies ><
Thanks for letting me know.