Nylea's Disciple is a 3/3 creature with the effect of "When Nylea's Disciple enters the battlefield, you gain life equal to your devotion to green." To figure out your devotion to green, you count the number of Forest (green) symbols in the mana costs of the permanents you control on the field. Here's a link to the card on the Magic site if you want to see it in more detail: http://gatherer.wizards.com/Pages/Card/Details.aspx?name=nyl...
To get this achievement, you need to have enough permanents (creatures, artifacts, enchantments, etc.) on the field which have a combined total of at least 8 Forest symbols in their mana costs. The reason you only need 8 on the field is because the two Forest symbols in the mana cost for Nylea's Disciple DO
count towards the 10 you need for this achievement. I can confirm this because I played Nylea's Disciple with only 8 Forest symbols among my permanents, and when I played the card, the achievement popped. So again, if you have 8 Forest symbols among your permanents on the field, you can play Nylea's Disciple and pop the achievement.
An easy way to get this achievement if you don't have someone to boost it with (and some of the other achievements that require you to do specific things with specific cards) is to create two custom decks: one with the card that you need, and another that consists of only sorceries, enchantments, artifacts, etc. that can't affect you in any way. Then just go into single player, play a two player Practice Duel, and give the AI the latter deck. This way you have all the time you need to set up the field for the achievement you're going for because the AI doesn't have any cards that can hinder/harm you.
Also, I'm not sure how the booster packs work in the campaign (whether the cards in them are random or are the same for everyone), but I got Nylea's Disciple by beating the Ocean's Might battle in Theros.
Edit: Added a video guide for anyone that does better with visual guides instead of written ones.