If you activate Chimeric Mass’s last ability while it has no charge counters on it, it will become a 0/0 creature and be put into its owner’s graveyard as a state-based action.

Activating the last ability while Chimeric Mass is a creature will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.

If the number of charge counters on Chimeric Mass changes while it is a creature, its power and toughness will change accordingly.

For example, say you activate the last ability of a Chimeric Mass with three charge counters on it. After it resolves, you cast Giant Growth targeting it. It’s now 6/6. Then Diminish (“Target creature becomes 1/1 until end of turn”) is cast targeting it. Once Diminish resolves, Chimeric Mass would be 4/4. Activating Chimeric Mass’s last ability a second time would make it 6/6 again until end of turn.