If you're new and struggling, Mordred is usually your best bet. If that's the case, it's a bit too early for any Relic for you, really.

Avalon's guidance is a very poor EX skill. It sounds nice, but 20% Atk+ is nowhere near 20% more damage done. Go with a debuff - if you were the guy with Sol, Mordred with Ambush should do you fine until you start approaching endgame.