Shadow Steelix

Steelix is a defensive juggernaut, and Shadow Steelix trades a fairly large portion of that huge Defense stat for a teeny tiny bit of Attack. And while it's true that Attack is definitely the most important stat when it comes to raids, that boost is nowhere near enough to make Shadow Steelix worthwhile.

Best PvE Offensive Moveset

• Iron Tail usually beats Dragon Tail and Thunder Fang on offense due to STAB.

• Heavy Slam shares typing with Iron Tail and outdamages Earthquake, Shadow Steelix’s other STAB charge move.

• Shadow Steelix’ offense is bad, and Crunch will only make it worse.
• Breaking Swipe is great, but not on Shadow Steelix.

• Psychic Fangs is only meant for PvP. 

Best PvE Defensive Moveset

• Dragon Tail is less likely to be resisted by Shadow Steelix’s usual counters.

• Thunder Fang can hit Water-type attackers for Super Effective damage. 

• Crunch can be used frequently as a 3-bar move and is hard to dodge, but is resisted by common Fighting-type attackers. 

• Psychic Fangs and Breaking Swipe have pitiful damage on defense, making them worse than if Crunch would be resisted. 

• Heavy Slam is slightly slower as a 2-bar move, but deals more damage with STAB. 

• Earthquake is a joke on defense.

Dragon Tail is preferred for its neutral coverage and slightly above average energy gains. Thunder Fang is more of a niche option due to its worse energy generation, but provides a different coverage option. 

Psychic Fangs is often the preferred Charged Attack, providing Steelix with consistent debuffs and some coverage against Fighting-type Pokemon. Breaking Swipe's Attack debuffs synergize extremely well with Steelix's gargantuan bulk, but it is rather inconsistent. Crunch costs a moderate amount of energy and generally provides good neutral damage. Earthquake is very valuable for its wide coverage, and also gets STAB. Heavy Slam gets STAB but Steel-typing isn't very valuable offensively.