it should be easy to kill, not only when it comes into game
the first nerf was very good, but it still need something else
two things that also make it very strong:
it becomes very hard to kill after using it first time
if it takes only 2 damage, you can destroy another permanent
i don't know if the black hole is something good or bad, the shard may get a little weak without it
it could be something like what thispersonisagenius said:
shard of focus 7

Destroy a permanent and gain +0/+15. Turn into a black hole if HP>25 (HP>35 if mark is gravity)
0/1
2 permanents destroyed, 3 if mark is gravity
shard of focus needing more than 2 damage to give you a chance of destroy