[Dragar]

I'd just rush him with H3s as soon as possible, catch up on research afterwards. Raise dominion as much as possible when they aren't actively attacking, especially if sitting in conquered lands he may take back as you fight. You will need supply items to attack if you don't take him out quickly.

The longer the game goes the more useless his land becomes to you, so don't hesitate.

Attacking raids can be useful too if facing a defensive force to big to handle. It normally takes a while for his troops to reach you, and few scripted banishes then retreat can wear him down or force him to attack into your lands, which lets you fight in your dominion and avoids supply issues until you are ready to crush him. The lack of healing on undead could make this especially effective over time.