Asguresh

Independent: During a combat, Asguresh can cancel a combat card played by the opposing minion as it is played, and its cost is not paid. Scarce.

Can Asguresh use his ability even od He doesn't take part in combat?

And how many times can he use his ability per turn and per combat?

1) no,text say:played by the opposing minion
2) 1 time per every combat-turn not important,key words is:During a combat

This is a clear example of "during x, do y" mechanic, which can only be used once (each x, in this case - combat).