When I first started to play clash royale, I got the witch as my first epic card. It was really powerful in lower arenas, with

OP Witch

the ability to spawn skeletons. The thing is, I didn't realise that it actually cause splash damage. After knowing it, I read card descriptions more carefully. 

Tower wrecking Prince

The Prince is a really scary unit in lower arenas since it can wreck a tower. Furthermore, it's hard to counter a prince without cards such as the skeleton army and tombstone. After that, I learn that I can actually counter a prince with a witch, or even the giant plus support troops. The Baby Dragon seems dangerous but actually it does not. The only counter that I know to it is the musketeer at that time. Then I realise that Archers and Spear Goblins can counter it by using the arena tower as a tank. 
Baby Dragon

Tough Baby Dragon

Finally, the skeleton army is the scariest troop in Arena 1. The only known counter to it for me is the bomber and arrows. If I don't have them, I would end up losing the game. I really enjoy the feeling when a bomber bombs the skeleton army since it saved my towers life.

Skeleton Army

Scary Army