Maybe your heroes aren't leveled up enough? I maxed out this pair in every mission, and in the last mission, I mainly fought using magic. Two or three Meteor Shower spells, and they were all dead. The particularly tough ones got hit with Implosion. By the end of the mission, they were already level 30. The demon at the end only managed to attack twice. Opening the gates was difficult, as the drainers completely stole the magic, but they gave a lot of experience for it, they didn't hold back. I initially jumped over the gates and then bought artifacts in the cart (southwest of the map) that increased initiative, and it became much easier to fight the drainers. And in the very first battle, I won through trickery: I lured their strongest hero, Shade, out (taking all her troops) and led her to the gates as bait (while Raeleth was supposedly busy destroying the city in the northwest), and then I started running away from him towards the starting point of the mission. Raeleth ran very far in one turn and caught up with him before he could kill Shade. In general, if you don't do this, it will be much harder to play later. I had to replay once because if you don't take him out right away, he'll gather troops from three cities and chase you around the map, and it will be problematic to get Shade to the lower gates. Take out the scoundrel while he's young!