I NEED HELP I DON'T KNOW WHAT IM DOIN In all the other Pokemon games..

You have a well rounded team by the looks of things, to me it seems like you should be able to beat the Mauville gym leader. Taillow evolves at lvl 22 into swellow, aron evolves in lairon at lvl 32. grovyle wont evolve until lvl 36 into sceptile and to eveolve your kadabra you need to trade it. I would either train you Pokemon or keep fighting the leader until you win.

For starters I have to ask the obvious things like have you been battling all the trainers you can? Because even in gyms you should NOT go around the trainers. Second, if it's the Mauville gym you're stuck on Aron, who evolves first at lvl 32 then at 42, should be good at damaging them with mud slap.

Grovyle evolves at 36 and kadabra evolves through trade. I beat that gym leader with a Geodude (electric does not effect ground). So just carry some potions/super potions and a couple paralyz heals and SAVE BEFORE YOU FACE HIM!!!
This saves you from losing a load of money.
Hope I helped

See when I was at the third gym leader I only had 1 Pokemon still, because I trained him really good we always won and he was water, and at the 3rd gym leader we creamed him even though he was a electric trainer. So thats how I would do it, keep your one best Pokemon and train it really well and at like the 5th gym leader get another Pokemon and train both of them really good. Thats just how I would do it.

You've got a good team. If you are able to teach any of them a gound type move then you would kick the gym leader's @$$. Aron evolves in the 20's. Taillow evolves on level 22. Grovyle evolves on level 36 and Kadabra evolve when you trade it.