You attack one at a time. Inigo + Cordelia combo is a great example of this - tap Inigo to buff Cordy, then tap and attack with Cordy. If she gets a kill, BOTH her and Inigo untap - allowing you to attack with Inigo, then Cordy, and if Cordy's second attack kills, Inigo untaps for his own second attack attempt (or to tap and buff another girl).
Nothing's stopping you from attacking the opponent's MC every turn (besides attack range) with every unit - but if you leave the opponent's board up AND buff their hand size by breaking orbs, they are definitely going to have big turn once theirs rolls around.
I found that attacking the opponent's MC became less and less useful the better our decks got, but nothing is stopping you from building an aggro deck around the idea that you're attacking MC every turn and trying to end the game in the first 3 turns.
