Now this is where things get started
Steelix prob have the Dura advantage. He also can hit hard with Stone Edge and Rock Blast, with Screech there to help with the damage, with curse to help him boost his' defense and attack, while lowering the speed. Steelix can also use sandstorm to do chip damage in Cloyster. He can also paralyze cloyster with Dragon Breath. Steelix also have sturdy, that can save him from a KO with full life
Cloyster prob have the speed adavantage, either that or both speed are equal. The AP is prob equal to. Cloyster have the type advantage, and can hit hard with STAB Hydro Pump (assuming that STABs are allowed here). A thing that Cloyster have over steelix is the Stat boost, with Shell Smash boosting more than curse, but the defenses lost can hurt Cloyster a little. However, the boost would prob be enough to take in Steelix, with Cloyster having the speed and AP advantage after one Shell smash boost. Skill Link won't come here since Hydro Pump is prob the only move that Cloyster will use to give damage, if the wiki allow Hidden Ability, then Cloyster wouldn't even care about Sandstorm with Overcoat, or cancel the critical hits of Stone Edge with Shell Armor
Overall, i think that Cloyster get this with Mid-High Diff. He have a better Stat Booster, that even with the def drops, he can prob live one hit of Stone Edge, and then abuse the speed and attack that he gained, he can even bypass Sandstorm with Overcoat. The way i see Steelix winning this is if he paralyze Cloyster with Dragon Breath, but even then, Cloyster would still be more powerfull, and be equal in terms of speed with Steelix even after paralize because of the stat boost. Another way would be if steelix get a crit with Stone Edge (that have high critical ratio), but stone edge can miss, and the crit chance is not that high even with higher critical ratio
