That's the developer's console that helps you to debug the game if you run into issues. You can prevent it from popping up automatically by going to the Plugin Manager > double-clicking on Yanfly Core Engine > under the "Screen" parameters on the right-hand side, you'll see "Open Console". Just set that equal to false.
Note that you can also open this console when playtesting by pressing F8. Errors in your game are reported under the "Console" tab.
Edit: I just noticed that you posted this under MZ but you say you're using Yanfly Core Engine (which is an MV plugin). Anyway, if you're using Visustella Core Engine instead, the steps should be similar.
I've moved this thread to Plugin Support. Thank you.
Also check the Console tab and see if there is a message there. Even if you have that option turned off, the plugin might force it open if you have an error somewhere (like an incorrect version or a missing plugin that's also required). Just make sure it's not there for a reason first.
It makes sense that you try to focus on the squishy mages at the back of the party first, right? If the mage can out-DPS the enemy DPS before the latter can kill the tank in front, the fight becomes boring.
…I think I might need to switch from standard ATK/DEF up/down buffs to parameter altering states. I just had an actor whose usual attack against an enemy does ~25 do a buffed attack against a debuffed enemy for ~600…