I don't think the Warlord is supposed to only swarm. What about his defenses and outposts? Archer's Watchtower is made for wall users. What about his panzerguardes? What about Thorg?

The priestess is best with turtling, and the beastmaster is best with swarming, but I'm sure the Warlord is meant to do both. What would make this possible is zone control. If you control more territory, you can adapt to threats more efficiently by moving the appropriate creatures to offensive or defensive positions with less hindering. I think while the Warlord needs to support his soldiers, he ideally should never have leave the territory he controls unless it conquers more territory before he can get hurt. Traps could be placed on the outskirts of your territory to dissuade potential invaders, or to punish those who do attempt to invade.

His many goblin soldiers are very weak and die easily, but I suspect that the Warlord gets them to swarm by summoning a LOT of them and at least partially protecting them and clearing a path for them. Like casting shift enchantment to move a fortified position ahead a couple zones to help clear a path, then casting his standard bearer on a powerful creature to lead the swarm, probably the warlord himself so that he could use battleorders on his swarm more easily. Because of that I propose that the Warlord himself should be a soldier creature. Afterall, he probably didn't inherit his forces by birth, he probably had to kill another warlord to achieve power (I'm pretty sure that's the way it works). Not to mention he's an Orc wearing a lot of armor and nasty weapons. Maybe I'm exaggerating his soldier-ness.

If the warlord is not a soldier creature then another powerful creature will have to lead the Warlord's troops to victory (like Thorg), and the Warlord will absolutely NEED his Horn of Gorthos to win this way.

If you have a means to protect the swarm so enough of them can survive to attack the enemy mage, you have enough attack power in your swarm and its leader to kill the enemy mage, and you're not in danger of being killed yourself first, then that is the moment to march forward and take 'em out. Think of winning "Tower Defense" style, except that you're the one sending waves of enemies forward.