You only encounter maxed promo units, maxed fragments or powerful rewards from arena challenge or boss strike? And you can't beat them with your normal units?
No problem, you need other arena defenses you can beat without losses!
In the Arena a (hidden) score exists. This score controls the player/defenses you encounter. With a higher score the enemys are stronger, have higher level and the units have higher ranks. The key to more wins in arena is to lower this hidden score by loosing some arena battles - don't bother if you find enemys to strong there's no drawback for you on the long run. To lower the arena score place some units like wimp, trooper and arsonist (no matter which rank, low healing cost is all what's important), fire one shot and retreat. Firing and retreating is important to lower the score, only retreating without firing keeps your score - and the strong enemies. Repeat this 6 to 9 times and you will encounter less challenging opponents and can then start to really fight and beat them. Win the battles until you find defenses to strong to beat and start lowering your score again.
The hidden arena score drops faster by loosing than it raises by winning arena battles. Usually it takes about 15 wins until to the score is on the starting level.