Don't use your energy for battling, unless your trying to increase your battle stats. Use it on the plains. Each monster there gives anywhere from 6-22 experience, and also 0-3 gc. You can also find credits, gc and other secter things.
PS: Check out 22,12!!!

Buy GC, checks or tokens when someone sells them for low prices at the market, and sell them for higher to make more money.

Buy stocks, when they are low then sell it when it is high.If this is done right, you can become rich very quickly.

Buy clan raffle tickets. You may befome luckly and get 100k in credits!

Gaining levels/Battling:
If you want to go up in the levels fast, instead of just battling people in the 'Battle Arena', go to the plains and wander around there. It only costs you .1 energy to move around and you will find monster pretty frequently there, which give 8-29 exp. They also give GC, which can be sold for 50 credits each in markets.

If you want to go up in levels, you will want to have strong mechs. I reccomend buying a upgrade pass as soon as you can, and spending at least 3-6 energy a day (The rest sould be used in plains) on traing your Agility, Strenght and stamina. Try to work on all 3 instead of only one.

Getting more than one mech can sometimes give you the overall battle advantage against other players. Having more than one mech means having a better chance of hitting morand giving more damage. Try to train both mechs.

Try to upgrade your weapons and armour every so often. Then can sometimes add than boost of attack or defence you need to win.