Actually 10 gold athena provides 6630 coverfire, which get split amongst you cards (1326 each)
All 6 star cards have a base of over 4.7k stats, max of over 9.8k and gold of over 12.5k. They can't be beaten by coverfire alone.
However, there are some quite potent 4* cards than can compete with 6* cards if you build your crew right, as well as 5* cards that completely outclass every other card if your crew is built right.
Alas, unless your 3* cards are gold and the 6*'s are level 1, there's almost no chance of beating those cards.
Problem I find is I don't know how it chooses what provides cover fire.
Coverfire is determined by the 10 highest stat cards that are not on your crew for that stat.
For your attack crew, it is decided by the 10 cards with the highest attack that aren't on your attack crew, and is 5% of their total combined attack.
For your defense crew, it is decided by the 10 cards with the highest defense that aren't on your defense crew, and is 5% of their total combined defense.