Monastery Mentor is even more powerful than Young Pyromancer. Monastery Mentor has prowess, so it's a threatening presence by itself, but the tokens it produces also have Prowess! Each spell cast will not only grow the ranks of token army, but it will also temporarily grow the size of each individual member. Monastery Mentor scales wide and it scales up, and it's exactly this sort of compounding growth that makes the card so appealing and worth building around.
I have to share two Modern brews that focus around Monastery Mentor.
Jeskai Monastery Mentor Delver
My first thought for using Monastery Mentor in Modern is putting it right into a Young Pyromancer shell. Young Pyromancer is an integral part to the Treasure Cruise-fueled Delver of Secrets // Insectile Aberration decks that currently dominate the format, and these decks are well-suited to utilizing Monastery Mentor.
Yuuya Watanabe found success at the 2014 World Championship with a UR Delver deck splashing Green for Tarmogoyf. I adapted his decklist to incorporate white and add Monastery Mentor, which replaces Tarmogoyf as a powerful piece of board presence and adds redundancy to the Young Pyromancer plan.
The move towards white opens up the sideboard to some of the best options in Modern. I have included the oppresive Stony Silence to combat Affinity and Urzatron strategies. Aven Mindcensor stops the namesake card in Birthing Pod strategies, while Hushwing Gryff is an interesting option for combatting their multiple come-into-play effect creatures, including Siege Rhino.
For a completely different take on Monastery Mentor, how about applying it to a BW Tokens strategy?
BW Monastery Mentor Tokens
Monastery Mentor mentor produces creatures tokens, and these tokens are abusable just as is any other creature token. BW Tokens has been a fringe Modern players for as long as I can remember, and perhaps Monastery Mentor is just what it needs to be pushed over the top and into the spotlight. Monastery Mentor gives BW Tokens a powerful new tool that may allow it to compete with the heavy-hitters in the metgame.
Unlike Young Pyromancer, Monastery Mentor and Prowess trigger from all noncreature spells and are not restrained to just instants and sorceries. This makes Monastery Mentor particularly excellent with the enchantment anthem-effects available to to the tokens strategy, because these enchantment will not only grow the token army but also trigger Monastery Mentor and the Prowess on all of the tokens.
BW Tokens has been an unfortunate victim of Treasure Cruise, but it's an archetype that has a ton to gain if there is a banning later this month.