Stalwart Successor + Phyrexian Altar + Grazing Kelpie

Stalwart Successor

Phyrexian Altar

Grazing Kelpie

How it works

1. Activate Phyrexian Altar by sacrificing Grazing Kelpie
2. When Grazing Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. When Grazing Kelpie enters, Stalwart Successor triggers, putting a +1/+1 counter on Grazing Kelpie
4. Repeat.