I'd say you should just check the videos and try. most event maps have tons of life so you don't fail the map all that easily, just leak a ton of monsters and lose out on drops, getting a 1*. also, http://millenniumwaraigis.wikia.com/wiki/Event_Missions you might want to read up on event missions to get a better understanding of each type of event mission so you know how far you want to go yourself. as a rough estimate, you need cr2 on an event unit for it to cost the same as a premium gacha unit of the same class.