Spin as many gym disks as you can. Especially gold badge gyms if you have any. A good start would be to make sure the first spin of the day is a gym. You can also search for quests that give healing items.
Additionally, learn to dodge, slaking shouldn't do much damage if you dodge it's charge moves.
I Need Healing (Items)
I recently had to knock out a slaking (Raid happening there, ealy morning, guy already got his coins) and was golden razzing it long distance, and having to knock it out 21 times depleted all my healing items, well all my potions and max revived, Im fine on revives, any idea what the fastest way to restock them is?
TL;DR, out of potionsmax revives, and need to get more fast, how do I get them?
Answers
Healing items is always a problem for me. I try to hit only demotivated gyms, and most of the times I stop trying to take it as soon as I see that someone uses GRBs.
Fastest way to get potions: Raids. Lots of them. I usually raid a lot to stock up on potions then stop raiding as often until I am low on potions again. You can also get the max revives spinning gold badge gyms that are your teams colour.
Like said already, spinning gym disks is the way to go to get more. If you have any spot with 3+ gyms nearby each other you can go walk a loop around them. Spots like that are great targets for getting gold gyms since it makes your grinding for healing items more efficient.
Other solutions:
Dodge more. You said in a comment that you have problems dodging but even lower quality phones can dodge, it's just harder. Always time your dodges based on the yellow flash, it's the most consistent method to get the timing right. The first sign that a defender is using a charge attack is the battle log at the bottom right corner, the text for it will appear instantly and warn you to look out for the flash/attack animation.
Use disposable attackers. If it has 2k+ cp, it can probably fight adequately. Catch a high level base form of something, evolve it, fight with it, trash it. It'll be slower than the top tier attackers, but it won't consume your potions.
Use more potion-efficient attackers. Lugia, Aggron and Ho-Oh are great for this, but the exact list depends on what you're fighting. High defense mons with resistances.
Raids. Huge group raids where you can go in with disposable or tanky mons without worrying about your damage contribution are likely to be a big net positive, but tier 1-2 raids also tend to give more healing items than they consume.
I sadly do not have a lugia, didnt start raiding t5's until latios, didnt know there was a raid chat where I am.
I do have a strong aggron, 98IV, attack hp, but he's always low hp, and I also have a ho-oh, but same story.
I go out for t1/2 raids using that strategy, and it does work, but my item space is limited, 400 slots and half are taken up by GRB and RC (I only spend coins on event boxes and incubators these days, if the bag/storage upgrades were cheaper, id get them)
Consider dumping your rare candy on Kyogre to make space. It's a good target and hoarding RC with that little storage space doesn't make much sense. As for GRB, they're there to be used. If they start to accumulate, you may want to use them on difficult wild catches to make catching faster. They're not something most players should hoard.