In the dragon slayer (at least in my version), the following tactic helped me: in the first mission, you get a built-up town with a library and level 5 magic. You need to restart the game until you get Resurrection, Explosion, and City Portal available at once. This is very helpful. In the other missions, the portal is not available. Then, by the second mission, you can recruit an Earth expert, and you can choose the mission with the Rust monsters. This makes reviving units easy. Keeping the last mission in mind, it might be worth taking Mysticism at the beginning. It makes things much easier.