Wildborn Preserver
Creature — Elf Archer
Flash (You may cast this spell any time you could cast an instant.)
Reach (This creature can block creatures with flying.)
Whenever another non-Human creature you control enters, you may pay 
. When you do, put X +1/+1 counters on this creature.
2/2
Illustrated by
Foundations#650RareEnglishLegality
Rulings
Oct 4, 2019
If you pay while Wildborn Preserver’s triggered ability is resolving, the reflexive triggered ability triggers and will resolve separately. This is different from effects that say “If you do . . .” in that players may take actions after you’ve paid mana but before the creature gets +1/+1 counters.