>>538260880
They do similar but pretty distinctly different things. Not exactly interchangeable.
Hellagur is more suited for elites that hit hard enough to get his HP down, while Mountain with his upped ATK and multitarget from S2 combined with his crit talent lets him shred medium sized fodder in all but absolute swarms.
He's also much cheaper, 11 to 9 dp, and is even an alternative to having a vanguard on some teams and stages.
His best trait in my opinion is how much damage he can soak with S2, 6% HP regen a second with around 3k hp is no joke. Basically nobody in the game can soak the amount of arts damage he can for the cost of 10dp.