Hi @CodePost3 and thank you for posting this here.
Each league has a specific unit level cap. In the highest league (Tiberium) units are capped down to level 15, so it should not be possible to fight against units higher than that anywhere. Lower leagues (master, Diamond, Gold, etc.) have lower level caps. You can find all the details on the battle map screen (tap on the faction) at the top right corner (the little "i").
What is possible, is that you meet players who have a much higher player level (current cap at 60), but that does not affect the unit level cap itself during a battle, only what units they have available. If you are matched against much higher level players, that only shows that you are good at the game, so don't give up. Each battle, unit upgrades etc. provide you with XP, so you should be able to acquire new units soon.
Thanks again for your honest feedback, we always appreciate that and are constantly fine-tuning the game to balance it as best as possible.