Here's the reason that he is a mythic rare: as long as he's on the board, you can never take damage that will bring you below 7 life. Unfortunately, he doesn't prevent effects that make you lose life. The card effect has to specifically deal damage, not make you "lose life" and that's the kicker. So, Elderscale Wurm was a marginal Standard play at best. Still, he had a lot going for him.
He's a 7/7 trampler, which is quite good. When he enters the battlefield, if your life total is less than 7, Elderscale Wurm restores you back to 7 life. The other good news is that you basically become invincible to combat damage as long as he's on the board and you're at 7 life.
This guy was a definite bomb in Limited. But in pretty much 60-card Constructed format, he was mostly never considered. However, he's quite good in Commander. While he's never really found a home in any particular deck, Mayael the Anima can cheat him out and Omnath, Locus of Mana makes so much Green mana that the Wurm is easy to cast.
Elderscale Wurm is a good mythic rare and another good green fatty for the mono-green lovers out there. Also, he's got 7 mentions of seven (including his power and toughness) on his card. You got to give the design team props for that. This card wins in both terms of flavor and being a Limited bomb & Commander powerhouse.
Have you ever played Elderscale Wurm?
~ Phoenix <3