As it is now, IMO: jelly king's minion spawn rate is simply broken. He spawns same amount of minions (not 4, more like 40) when you solo him and fight him in a party. In my opinion its simply ridiculous.
50 mini jellies have 50x damage and 1x hp when you go solo.
50 mini jellies have 50x damage and approx. 3x hp when the party is full.
See something wrong here?
There should be a 2-4 times smaller limit on how many minions Jelly King can spawn when you go solo.
And no, its not a suggestion. Its a bug report.
Learn to distinguish between a bug and a balance issue. Everything here is working exactly as the developers intended, so no, this is a suggestion, not a bug report.
Also, plenty of people have killed JK just fine in both solo and party situations. If you're having issues, you should consider changing your loadout and/or strategy.