Rest is a bad option. You'd have to use rest after 1 hit most likely. This thing can be 2HKO'd, though a lot of attacks will only 3HKO it. The problem is that if 2 hits take you out of schooling, you're essentially dead, and only a slow wish-pass would save you. That means you're resting at 40-60% HP. Unless you're going to switch it out at full HP and use aromatherapy, sitrus berry seems like the better option. Lum + Rest can heal for more than Sitrus, but you're using a turn to do it, a turn where your opponent might be setting up. The ability to remove status is nice, but this thing is a mixed attacker and can function through status.
% based healing isn't powerful because it's HP is so low. Aqua Ring, Leftovers, etc. don't seem like good options. For Shell Bell to be better than Leftovers, you need to do at least 152 damage in an attack. That's certainly possible, but does that translate into a significant amount of healing? No.
252+ Atk Wishiwashi U-turn vs. 0 HP / 0 Def Starmie: 204-240
A full-offense set with a super-effective attack, and that's only 30 HP max (just over 10% of your total) gained when you're taking 150+ (50% or more). You're winning the trade in terms of damage dealt, but you're still weak enough to get finished off in 1 hit.
0 Atk Wishiwashi U-turn vs. 0 HP / 0 Def Starmie: 156-184
Fully bulky, that's 23 HP max gained when you're taking 130+. You're still dying in 3 hits.
Best answer is to bring it in safely or on a resisted hit, U-turn for momentum or OHKO with his high stats and decent coverage, then pass him a wish later. His stats are fantastic for receiving flat healing.