The best strategy is to finish this bestiary on Rapid Respawn together with other bestiaries because it is a bit time consuming and can be finished with energybomb the easiest and fastest way while focusing on something else.
Go to Gray Island, Fiehonja and enter the hole east / south.
Then walk down the stairs and mark yourself the spots on your Minimap the way I did.
I also put some Stoneshowers on one SQM north (if possible) to the spawn sqm of the Jellyfish so I remember where to use the energybomb correctly (the right marks are the lips! The flags are spawning sqms of Fishes and Calamary if someone is also interested in this.)
In this place are 3 spawn sqm for Jellyfish. If you use the energybomb tactic I would recommend following way:
Start shooting your energybomb on the marked sqm's and repeat that every lap. While running the round you can kill Deepling Scouts and Deepling Warriors.
At this place you can do the bestiary for following creatures:
- Jellyfish (energybomb tactic)
- Calamary (energybomb tactic)
- Fish (energybomb tactic)
- Deepling Scout
- Shark (other spots might be better for this one, I have not finished it yet)
- Deepling Warrior (other spots are better for this one but you can gather a few at least)
This way I finished Deepling Scouts, Jellyfish, Calamary and Fish almost all at once and at the same time. It took me some less time for Fishes and some more time for Jellyfish but on Rapid Respawn it is maybe 10 more minutes for them. For these 4 monsters you make 60 Bestiary Points in total.
If you only want to make the Jellyfish bestiary I would recommend to do it the same way but just use engergybombs on the spots and then afk a for a while on the stairs. It is a easy reachable place and you can go afk at the stairs without beeing attacked by monsters.