Their main weakness is magic. However, I have not discussed this in this guide for a good reason. There isn't a spell that is effective enough to kill them one by one to make it worthwhile. Bloodvelds are simply way too large to effectively use multi combat spells like ice burst.  Due to their weak defense against range and melee - even if it is a bit better than against magic - it goes way faster with those two combat styles. However, if you want to use magic, nothing is stopping you from doing so!