Is the only way to level up though winning attacks?

You can spend points too.

Upgrades>Use points> Wise master advice

only thing with spending points for it is during low lvl it'use' to work well but now everythings changed and all so id say you'll probs be better off keeping them to upgrade other things in point store and/or selling to players to buy new gear... but yes winning attacks against players/quest 

Points give you more exp than round 3, but I know what you mean.

I am considering other ways to gain experience, actually Not until the maps are done though.

Awesome, good to know!