Combo with Karn 2

Whenever Karn blocks or becomes blocked, it gets -4/+4 until end of turn. : Target noncreature artifact becomes an artifact creature with power and toughness each equal to its mana value until end of turn.

This artifact doesn't untap during your untap step. : Add . : Untap this artifact.

Activated abilities of creatures you control cost less to activate. This effect can't reduce the mana in that cost to less than one mana.