So to start, his name was initially Illuyankas but was changed to Jormungandr because he was another boss in the mod that was removed. Not sure why the name was changed? Maybe Sega liked it more, regardless, it's the same boss. Anyways, some help.
Yeah, use a Dirty Bomb to start. Slow also works so use that. He'll also be weak to Lightning so Bolt on your best Mage will do well. Chocobuckle, the Enemy Skill, is also great for this fight if you picked it up back at the farm. You can put your characters in Fury if they can't attack him because of Long Range to get more Limits and also put everyone in the back row to lower Tail attack damage. If you are using Barret you can give him Wind Bullets with Elemental - Choco/Mog junctioned together if you picked it up in Midgar.
Another thing to keep in mind is that if enemies are stringing a lot of attacks together, it could be a few reasons. A lot of enemies use counter attacks when they get below certain health thresholds, and some enemies just get to go multiple times in a row with certain moves. And I think Jormungandr can do both actually. If you're seeing a lot of attacks in a row, slow your attacking down some to ease that problem up.
If all else fails, you can search me up on Youtube where I uploaded every boss fight for anyone that wants/needs help. Maybe some visuals could help as well. The video will be named Illuyankas but it's the same fight as I mentioned earlier.
Just watched your video. You're using the exact same tactics I am using apart from the fact that I am just focusing Cloud and Yuffie on bolt magic with Aeris being healer/buffer.
Just tried it a couple more times with the last attempt going as follows:
-Match starts, ATB gauge fills for Yuffie
-Go items and scroll to pick spider web to inflict slow
-During this (ATB is set to wait), Jormungandr uses 'Serpantarious', doing roughly 400 damage and inflicting Stop on Yuffie
-Instantly followed up by a tail attack which hits for 500 and kills Yuffie
-ATB gauge fills for cloud
-Jormungandr uses 'Cyclone' then 'Tail Attack' killing Cloud before any action can be taken
And after that I thought there was no point even trying that fight.
Couple more things to add about my game after watching your video.
- I'm playing the latest 1.5v on 'Arrange'
- Boss' name is different
- Texture seems to be missing for the body (all black)
- The damage I'm taking is roughly twice as much as you are taking (with all members in back row and barrier cast)
- Dirty Bomb almost never hits (don't know how my first one actually hit)
- 'Tail Attack' heals boss for damage taken and is only used against one character (usually as a follow-up skill)
- 'Serpentarious' skill being used which inflicts 'Stop' and a number of other status effects
- Never seen the boss 'basic' attack, thought it was all skills XD
- 16k health (don't think you ever sensed the boss in your video to see his health)
- 'Selenostrike' does health damage not MP damage
I've not had too much difficulty with the encounters after figuring out their weaknesses but this boss makes me think that the AI has been un-intentionally tweaked with the removed boss you mentioned. The battle itself feels like the boss is bypassing the turn order and unleashing mid-game skills and combos.
Just spotted the hotfix below download
'-) Arrange Mode: Bottomswell replacement boss fixed'
Ummmmm... Maybe that's why I'm having so much difficulty XD will report back once I've patched.
Ok. Nvm. It gives me the message 'Nothing to do! Files are already updated'. But I wanna say that this hotfix for the bottomswell boss isn't actually implemented in the new release.