I just noticed how on the 'in development' page, the shard now gets +15hp each use instead of +10, and also costs 6 unupped instead of 3.. This is definitely a step in the right direction. Another thought I had was to simply change the starting hp and hp gain of the shard, such that it can still be used, say, 3 times before passing the hp threshold, but have low enough hp that it can be realistically killed by thunderbolt or shockwave or something.
Something like this:
Starting stats 0/4
Accretion: Destroy a permanent and gain +0/+4. Turn into a black hole if it reaches 15 or more hps.
This would allow things like shockwave, otyugh, fire bolt, or even less obvious countering methods like blessing or chaos power to work. The flipside of this is you could do things like use eagle's eye on it to damage it so you could keep using it indefinitely. This would be fixed by the following change:
Starting stats 0/4
Accretion: Destroy a permanent and gain +0/+4. Turn into a black hole if it reaches 15 or more -maximum- hps.
I think that this prevents any abuse by things like eagle's eye, red nymph, aflatoxin, etc, to 'refresh' the number of permanents you can destroy, and gives ways for several elements to actually counter the card. It does take away the catapult synergy, but gravity already has loads of synergy with catapult anyway. It also leaves the possibility of fractal, mitosis and rewind/eternity, but it does get rid of some of the other problems, and even in the case of something like mitosis, at least the creature is actually killable with my suggestion, rather than being uncounterable by anything short of rewind or a fire lance backed by 50+ quanta. A card like fractal is also still soft-counterable by something like nightmare or discord, and rewinds run out and eternities are destroyable, so I think this would help quite a bit in balancing the card.
One more possibility would be to leave the card exactly as zanz wrote it, but add the following line:
If shard of focus is the target of any spell or effect, destroy it. (owner still gets the black hole in hand)
This would give any element the possibility of countering it, it would prevent fractal/mitosis/eternity abuse, it would leave the catapult synergy, and it would actually be somewhat thematic. (the spell/effect representing some kind of break in focus) It would allow any element to have permanent control, but it would be unreliable at best with such a clause; you could kill it dead with antimatter, lobotomy, or blessing, to name a few, while the elements with natural permanent control (explosion, steal, butterfly effect, pulverizer) would not have their PC be overshadowed by shard of focus.