Okay, I bet some of you wonder at times while playing a game like Pokemon or Mario & Luigi "How the heck did they code in that status effect?!", I was with you guys.. But, it's not as fancy as it seems. Heck, I think it only took me a month to master coding in TSs (Turn Skips) and DoTs (Damage over Time) for a Turn Based RPG battle format. So I'll show you how to make a DoT today!'
So, go into the Instance Variables of your victim target, and add a new number based one, the number if for how many turns it'll last. For this cute lil' tutorial, let's name it "Burn", feel free to put in it's description "DoT" or whatever. It's your game, so I won't judge.
Next, hop into your code for the fight, add a event that checks if it's your target's turn AND if he or she has the effect in question (Greater than 0), than give it a "trigger once" condition, so the effect won't just insta-kill them right there and then.
And I bet you're wondering "How do I make the turn count for it go down?", that's why I used a number based variable! So, for example: After the foe takes damage, have it switch to your turn and subtract 1 from the variable that's the effect in question.
Now, since we're coding a DoT, put some actions that cause the target in question to take damage and then go back to your turn. If you like, you can use a Text object to have them say something, usually related to the effect they're suffering from. What I did with mine is make it so after they say something, they take damage before it goes back to the player's turn. (Mine's 1 vs. 1, so it might be different with yours.)