Correction - Angeling card doesn't prevent ALL damage. There are two parts to the damage to the caster of GC. One is the fixed kick back, the other is the kick back from damaging the enemy. Unless this was changed at some point and I didn't hear about it- the fixed damage will ALWAYS be there. Angeling or not- you'll take those hits. However, because the damage done BY GC is holy and therefor the knockback to the caster is holy as well, that damaged will be nulled. Still helps a lot though, especially when you're GCing larger mobs. x.x;;
I don't think def/ mdef will effect the knock back. I don't know it for a FACT, but I think because the knockback is based on the damage you do to the enemy, the damage you receive becomes a fixed amount. Right? I could be wrong.
Don't worry about strength so much. I think attack might effect the damage some..I'd have to check that out. I remember andre cards being a popular choice for GC builds. But for the most part, like Tenshi said, Int and Dex will be your main stats. Along with Vit of course. Int will effect the damage the most, and also because of the cost- you're going to need a lot of SP. The casting time is long, so more dex is essential. During GC, your defense is reduced to 1/3 too- not to mention you'll be damaging yourself. That's why I included Vit as a main stat. Survivability.
I also heard about a patch (Once more nerfing mobbing) that changed GC so if several monsters are stacked on the same tile- not all will take damage or something anymore.
Just something you might want to be ready to deal with.