Petra, Nepotist's Favour

Enlist (As this creature attacks, you may tap a nonattacking creature you control without summoning sickness. When you do, add its power to this creature's
until end of turn.)
When Petra, Nepotist's Favour enlists a
legendary creature with greater mana value than
it, put a +1/+1 and shield counter on Petra.
You may tap an untapped legendary creature you control with greater mana value than Petra rather
than pay it's mana cost.