You can use the archives to replay all missions that youy are passed, including the ones where you have to choose between 2. For future reference, for Hard on up, kill the zerg air. It's easier to handle only ground than it is to deal with brood lords firing on you bunkers and turrets, then having that damn leviathan showing up. Nydus worms are easy to deal with when you have a decent group of banshees and the chokes are easy to cover with plenty of tanks. Be sure to have plenty of turrets up for the overlord drop and get upgrades to keep your army alive. Keep SCVs at the bunkers and soak up extra minerals with mercs if you hire them. Btw, banshees also work well for killing kerrigan when she attacks.
wait, if you cant go back to your ship after char, then how is it possible to do the other 2 mission achievements?
edit; oh k
is there an armory there as well?
yes, but you won't be getting research or money to use. Taking out the nydus worms is a fun mission, but killing air is much more useful in the end mission. It's easier to concentrate on ground than to do ground and air.