Sentry Gun

When this card enters the battlefield place 2 +1/+1
counters on it
This card cannot attack while a player while there
are any enemy creatures in play
{t}This card deals damage equal to its power
to target enemy creature
Upgrade-when this cards upgrade effect is
triggered place 2 +1/+1 counters on this card may
never have more than 6 +1/+1 counters on it