At what level should I try to tackle Silver challenges?

I'm a level 13 enginner and still get destroyed on silver challenges... at what level should a party of 4 try a Silver challenge? and at what level a gold one?

With a good party of 4, I've found that anywhere from level 15-20 can handle silver without too much trouble. I typically do bronze until up to level 12ish before trying out silver.

As for gold, I don't recommend anything other than 4 well prepared level 20s.

It all depends on your team. I've gotten several good one where we were able to all survive silver while below level 9. If you plan out how to handle diff. maps and enemies and have a diverse team even gold is do-able in the lower teen levels. One of the tricks is not using up equipment on the lower rounds, even if you die on round 3 it wont effect the over all game. Strategy is the best weapon.