The moment you start to feel over whelmed you cast sleep to sleep the multiple targets and fight each one 1v1 to mitigate damage. Use manaward to further low damage to hp.

If they are immune or have become immune to over use of sleep use Freeze to bind them, step out of their range and engage 1 at a time.

You still have access to heavy its called Break but Blm benefits the least from it.