I can see this one being alright, Golems do 11 damage on their third turn out, plus they do 7 and 9 on the turns before that (no wait for damage to start). They are much more vulnerable to creature control though. Combined with high damage, this seems like it would be a pretty good answer to RoL/Hope decks (only defeated by RoF/Plague/unupped Spine Carpace).
The ability seems kind of tacked on though. It kills low HP creatures, but there is no further effect. Maybe an attack decrease for the turn Trample is used to deter you from doing it each turn, this way it only acts as a super heavy hitter or good creature control, and not both on each turn? I just feel like there should be some additional effect.
You could also consider changing trample to being able to take out creatures that have X% or less of Mammoth's health (e.g., 25% would mean creatures with 4 HP or less). Then there would actually be a reason to buff it (Plate Armor anyone- give earth serious creature control) or for opponents to target it even if they can't take out that beefy 16 HP. It would also discourage the use of Rage Potions (Fire/Earth/Gravity Mark with Golems). This would require my earlier idea of some drawback to using trample because with some plate armors it could effectively kill anything.