I am stuck at the Silver 1 mission, too. There are just too many powerfull Epics and Legendaries.

But imo the mission for beating the first stage of spiders dent was nearly the same BS.

You get that mission very early in game, just to realize: wtf? How am i suppose to beat that thing with its 10 annoying minions?

But arena system is very RNG, because you dont see what artifacts your opponent has.

My tactic is like this: 

1. Valerie - Shields to protect my team from lethal AoE Counterattacks

2. Medium - 50% Attack Boost and as Leader for Speed Aura

3. Warmaiden - 60% Def-Breaker + a little AoE but it depends on RNG if the def gets weaken or not

4. Kael - AoE to nuke everything away

This way i should get to Silver 1, but still it depends on what opponents you get. 

And maybe i should level up my arena team first, my Kael is my only 5 star champ, all the others are still 4 star.