Mega Steelix

Mega Steelix is not good for raids. Flat-out, it's destined for obscurity. However, it's not entirely useless. Long-lived Pokemon like Mega Steelix have the advantage of granting their Mega Boost to the entire team for a prolonged period of time, making them situationally very effective in raids. However, this will only work if the entire raid team works together to load in specific types at the same time. 

With the release of Primal Groudon, Mega Garchomp, and Mega Swampert, Mega Steelix's role as a long-lasting Ground Mega has been largely diminished. Once Mega Metagross is released, Mega Steelix won't have much of a combat niche anymore.

Best PvE Offensive Moveset

• Iron Tail and Heavy Slam make for Mega Steelix' strongest moveset.
• Earthquake is the move of choice if the goal is long-term Ground Type boosts, for the raid team but the lack of a Ground Type Fast Move is definitely a problem. The secondary pick here should be chosen based on the situation:
  • In entirely neutral scenarios, Iron Tail will tend to pull ahead.
  • Thunder Fang and Dragon Tail are slightly behind due to a lack of STAB, but both are perfectly viable if Iron Tail is resisted and/or either one is Super Effective.
• Crunch is all-around inferior.
• Psychic Fangs is a PvP move.