Second ability is better worded as "Target player cannot add mana to his or her mana pool until the beginning of your next upkeep." Just so they can't do anything on their next turn with mana.
Third ability works as "Rashoan of Jund deals damage to target creature or player equal to the number of untapped Mountains you control." It's better worded by naming the source of the damage and capitalizing the subtype Mountain.
Also, leonin were native to Naya pre-Conflux, which also allows for the RG color combination. So if Rashoan was pre-Conflux, renaming the card Rashoan of Naya, unless there's a plotline nitpick to do on your friend's part, would be smarter. It's just too bad that Ajani's got some competition on that front.
Consequently that would make ability 3 into "Rashoan of Naya deals damage to target creature or player equal to the number of untapped Mountains you control."
If he's alive and ascended post-Conflux I *suppose* it works as is, namewise, but even Rashoan of Alara (although the other two sound better)
the second ability wont do anything tho, planeswalker ability's are played as sorcery's, and next players turn starts with the upkeep. you should probably change it to ''target player cannot tap lands untill the end of his/her next end step.
just a quick side note xD sorry!
second: the art is great! and jund is awesome... mmm i love dem dragons there.
my friend yitzy is the Magic the gathering master a my school i just know some of the illustrators. which reminds me this is the second piece of fanart that could pass off as official work you have done. your abilities know no bounds. btw you ever tried playing with any unhinged cards? yitzy favors the goblin trooper