Sentry

Sentry deals 1 damage to every creature attacking you.
{4}{u/r}, Sentry upgrades to level 2 and deals 2 damage to every creature attacking you.
{4}{u/r}, Sentry upgrades to level 3 and deals 3 damage to every creature attacking you.
At the beginning of your upkeep, pay {2}{u/r} or sacrifice Sentry.

...and if that don't work, use more gun.