[Stardust]

I usually invest up to reserve in the mind tree too then I focus on getting Onslaught in the might tree. Its very expensive but definitely worth it because your troops can act faster and weaken the enemy before they attack you on first turn (especially useful against black dragon, necromancer and other dragons as they have high initiative and speed).
You can also invest further in the mind tree (tolerance) instead if you're a fan of undead (vampires, cursed ghost, skeleton archer) and don't want them to clash with your other troops.