for those wondering it is possible to kill the last shard boss ...I did it with the bowman.....I haven't tried with anyone else...I had 3 of the strongest swordsmen and 1 of the strongest archers....I used rune of oak to get him down to about 2,000 hp....then I stunned the boss with rune of stone and then switched to rune of oak and blasted until stun ran out and repeated...
If you forget which button you pressed last, look closely and you'll see that it glows more than the others.