Alright, what mana link does is uses all your piety to deal that much damage to all monster and yourself to a minimum of 1 hp.
What you can do is: At level 1, cast a bunch of spells and explore without leveling up. When you reach max or close to max piety you use this ability. Then with fireball in hand (which you will probably have with all your exploration) you kill some 7/8 that has 1 hp and gain a ton of XP. This will level you up and give you back mana, allowing you to kill some high level 9s and other 8s (which still have 1 hp) for more level ups and more fireballs. When your this high level you can quickly kill off low level enemies to push to you the next level and further take advantage of the free mana from leveling. From this point reaching level 9, if not 10 is pretty easy. And killing the boss with hopefully still a good chunk of unexplored tiles and full potions pretty easy. If you time it right you can start taking down the boss while it is still hurt from this effect (plus you have more piety from all those spells).