"Shell Side Arm deals damage and has a 20% chance of poisoning the target.
Shell Side Arm forecasts whether its base damage would be higher as a physical move or special move, and becomes the category that it predicts would inflict more damage (chosen randomly if they are tied). " That is the intended effect of Shell side arm but yet it just so happens that shell side arm has a second ability not supposed to be implemented. For some reason shell side arm ignores all abilities like sturdy and disguise (similar to photon geyser's ability to ignore abilities).
This is all the info I currently have but there are way more things to test this with, currently shell side arm is way more powerful then intended and hopefully can be fixed.