A 3/3 with lifelink is already pretty great, but whenever one of your Clerics dies (even by your own hand), you get to bring back a Cleric that costs less
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For a little peek into why Monastery Mentor was restricted in Vintage, we have only to examine the EU Vintage Championships before it was hammered

$54.99 Beatrix, Loyal General (0426 - Extended Art) Final Fantasy Variants $10.99 Raubahn, Bull of Ala Mhigo (0465 - Extended Art) Final Fantasy Variants $44.99 Quistis Trepe (0440 - Extended Art) Final Fantasy Variants $24.99 Ultima, Origin of Oblivion (0421 - Extended Art) Final Fantasy Variants $10.99 Balthier and Fran (0477 - Extended Art) Final Fantasy Variants $17.99 Jill, Shiva's Dominant (0438 - Extended Art) Final Fantasy Variants $29.99 Freya Crescent (0460 - Extended Art) Final Fantasy Variants $19.99 Shantotto, Tactician Magician (0507 - Extended Art) Final Fantasy Variants $14.99 Fang, Fearless l'Cie (0446 - Extended Art) Final Fantasy Variants $17.99 Torgal, A Fine Hound (0474 - Extended Art) Final Fantasy Variants $15.99 Seymour Flux (0452 - Extended Art) Final Fantasy Variants $12.99 Zodiark, Umbral God (0456 - Extended Art) Final Fantasy Variants 0 results found

That'll keep away larger sized dorks but smaller armies but larger going-wide ones like a token horde won't care to sacrifice a 1/1 Soldier