Pay 1 Force One of your units in any arena gets Shields 3 until end of turn. Play only if a unit has been discarded from that arena this turn and only if no unit is attacking.
Keywords
Shields
Shields X: A cumulative, static ability that means, "The attacking unit gets –X power against this unit." Shields only matters when attack dice are rolled. If you give a unit Shields after your opponent has rolled for damage, it doesn't affect the roll.