Activate Trazyn using Phyrexian Devourer's ability by exiling the top card of your library, putitng a number of +1/+1 counters on Trazyn equal to the exiled card's mana value.
Repeat step 1 as many times as desired, or until your library is empty.
Activate Trazyn using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any target.