Shelgon is physically defensive enough to avoid the 2HKO from Choice Band Hitmonchan's Sky Uppercut after a turn of Protect, and that fact combined with a lack of physical weaknesses gives Shelgon a nice defensive niche in the tier. While Shelgon's Defense is high, it has little in the way of a supportive movepool outside of Wish, but it certainly makes do with what it's got. The main reason you'd choose Shelgon over Lickitung as a Wish passer is its lack of Fighting weakness, while Shelgon's appeal over Flareon is its higher physical Defense and lack of Water and Ground weaknesses.
Protect is super nice for scouting Choice Band attacks, racking up Toxic damage, and guaranteeing Wish recovery, but it can be forgone for Roar, which not only is rare in the tier but also prevents the opponent from using Shelgon as complete setup fodder. Roar also pairs nicely with the defensive nature of this set by exposing the opponent's team and racking up Spikes damage. Body Slam is pretty weak, especially without STAB or Attack investment, but it's nice for a chance to paralyze, which completely cripples a couple of the Pokemon Shelgon can wall, namely Pidgeot and Raticate. There's not a lot available that can go in its place; Dragon Claw is available and will get roughly the same amount of damage on targets with equal defenses like Pidgeot, but it lacks a paralysis chance, will be significantly weaker against specially defensive targets like Hitmonchan and Flareon, and still leaves Shelgon walled by Steel-types like Mawile; however, the higher damage against Rock-types like Sudowoodo could prove useful if your team is weak against them.
While Shelgon's typing is better suited for a specially defensive set, as it resists key special types in Water, Fire, Electric, and Grass, it simply doesn't have the bulk to pull it off, so it's not a good idea to take Shelgon in that direction. Hidden Power Ghost can be used somewhere on the set to ensure that Haunter can't just switch into Shelgon for free, but Haunter isn't super hard to counter and Shelgon will normally be passing Wish to its teammates anyway. Switching up the set and replacing Wish and Protect with RestTalk is an option that gives Shelgon a lot of longevity, but Shelgon just doesn't have the defensive or offensive presence on the battlefield to really make it worth it in the long run.
Spikes support can really make or break the effectiveness of this set, making Glalie and defensive Roselia great teammates worth strong consideration. Bulky Water-types, particularly RestTalk Dewgong and Whiscash, will walk all over this set thanks to Ice Beam and the fact they won't mind being hit by status. Due to this, bulky Bellossom and offensive teammates to shatter these Pokemon such as Plusle and Haunter are needed. Chimecho with Taunt or Heal Bell can set up on this set pretty easily, making Dark-types good teammates as well.
Finally, defensive Shelgon is best used in a defensive core with specially defensive teammates to cover those offensive Pokemon it cannot beat itself. Flareon stands out from the pack due to its access to Wish, resistance to Ice, and good synergy with Shelgon all around; the only issue with this core is the fact they are both weak to offensive Water-types like Seadra and Huntail. Other specially defensive candidates include Kecleon and Dewgong, both of which enjoy Wish support and Shelgon's ability to take on Hitmonchan.