Is there a particular boss you're having problems with? If you stay behind a boss you should be safe (except at Foriel). Anyway, memorize the attack pattern, so you know when to attack and when to retreat! If you don't do that, then you can't fight effectively.
I recommend the use of the level 30 transform, since you can move around pretty fast and it's A-attack does pretty good damage, pierces, hits multiple times and its short attack animation does not leave you open for a long time after unlike other transforms.