Vs'ing high lvls

It's a complicated system. There are TWO triggers for a challenge match. 1 is levels, the other is MEDALS.

So to get a UNIT level challenge match, they need to have 1.5 levels on average on your army and YOUR army needs to be within 3 levels of the league cap.

The second more common challenge battle is the other player is 200+ medals above you. No other requirement.

Example 1: You can get into a fight with a guy with +1.5 levels on you and if you are under the league cap by 4 or more levels (which is the majority of players in masters), it's not a challenge battle. This prevents you from wedging yourself too high so all your games are challenge battles due to the challenge battle system making you never lose medals, but always gaining them. 

Example 2: If he has level 8 units with 3000 medals and you have level 10 units at 2700 medals, it's a challenge battle for you. This player might have a good deck/strategy and you might find him a more difficult opponent, so the game gives you a challenge battle due to medals.

If you don't understand any particular part of the challenge battles, feel free to ask me, and I'll try to clarify. 

All that said, 2 personal tips: 1) Try not to to rush into Tiberium before you are ready. If you are having trouble with the odd Tib player in masters now, you will see a whole lot more of them when you get there. 2) The Rivals Champions coming, you will ONLY play players in your league. So if you are in masters, you will play against other masters players for diamonds (and to get the jack pot, you need to be undefeated for 25 matches in a row) So forcing yourself into Tib before you are ready will actually HURT your progression within the game.