Depends, there are like 9? maps, each with different objectives, but usually you will end up running around the map with your big blob trying to clear it. It's closer to old fashioned comp stomps than a competitive starcraft game. You have 1 expo, that most people take ASAP and after that you just power units and upgrades into your final composition. The nice thing is that there's not much balance, so certain commanders can solo almost all of the map (Vorazun...) while others are a bit more limited, but unless you have a friend your partner's skill can be anywhere from a master to a baked potato. The game defaults you to hard and anybody who ever touched the competitive side will have hard time losing unless their partner is closer to the vegetable side of the spectrum.
You get experience, levels and unlock new units and upgrades, which turn slightly undertuned opponent into a joke. The mutators are supposed to remedy that, but don't expect a serious challenge (maybe on brutal). Probably the only way to lose is to go full eco at the start and get swarmed by the first wave since neither of you has any units. On the other hand there is something relaxing about marching your maxed out army into the enemy's stronghold, popping all your abilities and watching the fireworks.