this.color.setGreen(true);
this.power = new MageInt(2);
this.toughness = new MageInt(3);
// {3}{G}: Target creature gets +X/+X until end of turn, where X is the number of creatures you control.
PermanentsOnBattlefieldCount amount = new PermanentsOnBattlefieldCount(new FilterControlledCreaturePermanent());
SimpleActivatedAbility ability = new SimpleActivatedAbility(Zone.BATTLEFIELD,
new BoostTargetEffect(amount, amount, Duration.EndOfTurn, true),
new ManaCostsImpl("{3}{G}"));
ability.addTarget(new TargetCreaturePermanent());
this.addAbility(ability);