In my opinion graviton mercenary is slightly underpowered. It cost 4
quanta to play and its stats are 3|5, and it has no ability.
This cards cost to damage ratio is quite low compared to other cards.
Comparisons:Hermatite Golem costs the same amount of quanta to play but It has 1 more HP and 1 more damage which makes it clearly better than the graviton mercenary.
Mummy costs the same amount to play but does 2 more damage and has 2 less HP. In most cases, 2 more damage is prefered over 2 more HP so the mummy is better than the graviton mercenary
Toadfish costs 1 more quanta to play, it has 1 less HP and does 3 more damage than the graviton mercenary. It also has an nice ability to come with it. 3 more damage, and a extra ability that is very decent is alot for just 1 extra quanta
Phase spider costs 1 less quanta to play yet it does more damage than it. It has a fragile 2 HP but keep in mind that it does more damage and costs less.
Lava Golem costs 1 more quanta and does 2 more damage more than it. It can also grow making it alot more powerful than the Graviton mercenary.