I would vote for cost / hp.
Smash just seems way too powerful with 1 cost, you can disable a permanent deck arch-type with a few smashing creatures.
Explosion is 3 | 2 to permanently remove any 1 permanent.
Graviton Mercenary is 3 + 1 per turn to permanently disable any 1 weapon/shield/shard. (Full flexibility only occurs upped and Raider does not get a cost decrease.)
Smash is more expensive that explosion and less efficient - the only advantage Smash has is in flexibility.
But don't forgot it is a creature itself, it still dealing damage, able to receive buffs.
A smash + a pillar, almost same as a 3/5 Creature with a Explosion, each side for 2 cards, with more flexibility.
A creature also has the vulnerability of being killed by CC.
Smash can only keep one permanent
temporarily 'occupied' - the moment you run out of quanta or Mercenary dies, Smash no longer works and the Permanent stands untouched.
A real creature with Explosion has the ability to
permanently wipe out multiple cards, not just 1 card.
Let's say a we had a "Pyromancer" with 3
, 3 | 5 , and
: Explosion.
After summoning sickness, Pyromancer could take out 1 or more permanents completely without having to maintain denial or worry about limitations - an unkilled Pyromancer could take out at least 2-3 permanents with repeated use of Explosion, and when killed, those permanents won't come back. In contrast, an unkilled Mercenary can only control one permanent at a time (the previous permanent comes out of delay), and when it is destroyed the Permanent can be used again. Mercenary also can't delay pillar/pend stacks, while a Pyromancer could slowly chip away at the opponent's quantum sources (said pillars/pends).
What about a 2/7 creature with acceleration | overdrive as an passive active ability?
How would this not conflict with Acceleration + Armagio? (On a related note, I tend to disagree with a few stat buff variants because unupped Mercenary treads very close to Charger in terms of role - certain buffs to stats that may raise cost as a side effect can make it overlap too much with Charger IMHO. When giving Merc stat buffs keep in mind that it should have a different niche from Charger.)