If your opponent tries to rules lawyer you, the untap effect isnt conditional on the discard if your opponent cant discard, the untap still happens
Take a look at the prices from MH2 (which was the most printed set ever until Tale of Middle-earth took it over last year), and youll see theres only a handful of cards there that are worth the price of a pack or more
They work differently to other cards and act more passively to gain lore or give abilities
Ancient Adamantoise If you can get to 8 mana and play Ancient Adamantoise, you will be incredibly well-protected
Even better, if you follow Nissa up by actually playing that Forest, and have seven or more lands, you can immediately flip her to her planeswalker half, allowing you to grab another land, draw a card, or make a 4/4