Here is the full list of all 8 Helmet Heroes achievements.
Have at least 1,000,000 on you at once.
Reach at least level 140 or above.
Complete the lost warrior achievement.
Have at least 100,000 on you at once.
Complete Joshua's mission and show him all Heropedia enemies.
Reach at least level 50 or above.
Complete the black hooded hatchet mission.
Reach at least level 10 or above.