I also defeated the legionaries guarding the spell. The idea is clear, in principle. The difficulty is on the hero level, by the way.
As for defeating the archers, I don't see any chance. There's only enough mana for 2 casts, and it's impossible to restore the missing mana with Spiritual Bond. It's simply impossible to potentially deal 150 damage. For the same reason, it's impossible to defeat the dolmen's guard. You need to add +1 knowledge at the start, and then it will be passable.