I personally like airIII for my archmage attack. It may have less damage than crush, but affect everything within two spaces. So, hidden shadows, another city, reinforcing units, units guarding resources etc. Air is weaker than fire at level 2, however.
As far as building more than one mana node, I only do when there is a spell (such as fireball, skeleton, etc) that I would like all mages to have. Since only 6-9 units will have the third level spells(unless you graft flesh), I never make enough nodes to give the third level free. It doesn't make sense to make extra nodes just for the few that promote to the last level; let them spend a promotion on it. Unless, of course, you focus on just one type of summon. You can make even level 2 summons very powerful with 5 or 6 nodes.
As far as building more than one mana node, I only do when there is a spell (such as fireball, skeleton, etc) that I would like all mages to have. Since only 6-9 units will have the third level spells(unless you graft flesh), I never make enough nodes to give the third level free. It doesn't make sense to make extra nodes just for the few that promote to the last level; let them spend a promotion on it. Unless, of course, you focus on just one type of summon. You can make even level 2 summons very powerful with 5 or 6 nodes.