Escar-Go-Go-Go

Once you pull the boss, 20 Spicy Lava Snails will spawn in the lava all around the room. When you click one you will get an extra action button that will let you throw it to the boss. Swim around and get all 20, with each one giving the boss a stack of [spell=411573] which gives him increasing damage, attack speed and move speed. At 20 stacks it'll be replaced by [spell=411581]. There's no duration to either debuff so if you're worried about the damage increase you could wait to start clicking snails until the boss is lower but we didn't feel the need and did it on pull.

I don't have a map with all 20 but I did take this screenshot and circled a few snails so you can get an idea of what to look for. They can be /targetted so if you're having trouble finding the last few try that. Most are within a couple seconds swim from the edge of the platform but one was a bit further and one was insanely far in the northwestern edge of the lava as seen in this screenshot. You'll want a really mobile and really tanky class to go get that one. We used a DH. Rdruid would also be great.

A couple classes excel at going out to grab them, and while this is by no means an extensive list some good class-specific tips are as follows.
Priest - You can stand on the edge and grip people back to safety after they get a snail.
Mage - You can Alter Time before hopping in the lava and then use it to snap back.
Warlock - Drop a port on the edge to port back to. Do not try to gateway as it falls to the bottom of the lava.
Monk - Drop a transcendence on the edge to port back to.
Druid - Travel form mysteriously treats the lava as water and you get increased swim speed.
Rogue - Shadowstep someone standing at the edge.

While you do want to get all 20 ASAP, don't forget about the regular fight mechanics as well. You need enough people left on the platform to soak puddles and circles to avoid the raid damage. Also keep an eye on the stomp timer before hopping in the lava because it knocks you regardless of range so if you're far out in the lava, you'll be pushed even further.

if the dev who named the debuffs for this fight ever reads this, your pun game is on point.

If you need help with the metas feel free to add me to Btag (NA only) or Discord!
Btag - FrostyShot#1667
Discord - yarini

Easy Solo for Worldsoul Saga Level 80-100 Pet Classes & Beyond...

1. Normal Difficulty.

1b. Achievement criteria can be tracked, will turn from red to white upon 20 snails being fed to boss.

2. Begin fight, aggro/misdirects to pet, place pet in the far corner. You, the player run to the opposite corner.

3. Boss eventually casts Catastrophic Eruption. Takes about a minute for it to go away. If you are far enough away, you won't get the stacking debuff.

4. Finally the eruption is over. /tar Spicy and head to the nearest Spicy Lava Snail located in the lava, should show on the minimap and Hunters can track them at least.

5. Grab the Spicy and head back to boss, all the while keeping boss aggro on your pet at all times. Keep pet healed and up as necessary. Throw the Spicy Lava Snail at the boss. Repeat 19 more times.

6. Criteria has been met at 20 snails fed to boss.

7. Nuke boss.

8. ?

9. Profit. Achievement Completed.

I found the picture in the comment above to be a bit too far out for the North West side. https://imgur.com/a/eRzJv7g

We ended up stuck with 19/20 for quite some time. However, there is one snail in the North East corner that doesn't show it's name plate unless you're on top of it, except for a split second when the boss is pulled. This was the one that we were missing.