[PvK]

How fast are your monsters? If the player has some hope to outmaneuver them strategically sometimes, they may be able to arrange for some attacks at favorable odds.

Another approach would be to try to figure out how the AI judges strength (it might be resource cost, or shield + structure, for instance) and then make the monsters seem tougher to the AI than they really are. For example, really high structure and powerful weapons but short range might be interesting - the player would be trying to keep away from the horde's attacks via fighting retreat, hoping to hurt them enough before they get cornered. (OTOH watch out that this doesn't mean hours of silly tactical battles where the monsters never get a shot off.)

Just some ideas...

PvK