ramsen yes, cleric could be useful, but you'll find he is not that important. the problem is in no mana regen. where he will find mana? just kill monsters faster and then regen rangers on last one. there is some guide in my profile
ramsen, it depends on campaign and difficulty rating. for example, to get beyond 200 waves in survival you will need both cleric and conjuror along with warrior in front. Only one slot left and I strongly suggest ranger. For regular campaigns at hardest difficulty cleric become useless and i'd suggest 2nd ranger, for their mana restore ability.
ClayMeow your tips all right for the badge, but if you want go further, you need to replace mage with cleric. Because 2 raisers will let you go beyond 100 waves.
Wargolems are bad. Cause they let Dwarven veterans hit back row. One alone warrior is good enough. Wargolems are waste of time and mana. I prefer my cleric hit enemies than heal weak golems.