what is the best shadow gun for jelly king?
i have the 4 star version of the winter grave, umbra driver and the grim repeater. I have tested them all on elite jelly king and my black chaingun appears to kill jelly king the fastest but that might be because of my uv, i was told that umbra driver is the best for jelly king and i have done alchemer switching and charge attacks with the driver. I'm not sure about the mega tundrus because the charge attack is kind of like the alchemer charge attack.

I'd say Grim Repeater is the best if you're planning on using just the one shadow weapon along with your team also using one shadow weapon; if you're using acheron or dark retribution it would be umbra driver/carbine/phantamos, if you're using a vortex it would be winter grave.
Umbra driver is mostly for doing longer range damage on charge shots, kind of like a sniper rifle. This makes it sub optimal on the boss, which is all close range combat due to the minis and spam. It would work and be mostly 'good enough' though.

I used the Shadow Blaster to beat the Jelly King but since that was the only good gun in my possession my opinion is now invalid. Phooey.

Keep in mind that the whole jelly Palace is limitating the usefulness of all Magnus-type guns.
I myselfe like to use my Mega Tundrus in various Levels and I also bring it to jelly, too. But here morely for the style instead of an Overall best weapon there.
These Little cubes just render the complete pierce-through-charge-attack-of-doom to one Standard bullet that deals a bit more damage to one enemy. Even with vortex you just hit a few while in the same Situation without These minniecubes you would slay everything in a vortex with one strike.
Conclusion: Wintergrave might be the best one if there would be no minnie cubes around; but since the Boss fight is filled with those, you have a high Chance to fall below your full dps potential.

Its a second for dps, from GR's sheer power
in the levels leading up to the boss there are plenty of non-mini rooms that make it extremely useful
and if you're even mediocre at aiming you can make it pierce through the boss, just don't be stupid enough to shoot the minis (this is why I mentioned crossfire)
1. Grim repeater: can one-shot boss phases, easily the best if you're good with it
2. Winter grave: can sometimes one shot, 2 or 3 definitely end a phase, freeze is good, pierces through the royal jelly dealing a lot of damage but make sure there are no minis in the crossfire (they're bugged and stop piercing attacks)
3. Umbria driver: has internal ricochets, but overall cannot pierce and has lower damage, really only works better in nonboss crowds outside the rjp, only if you're good with it though
I would recommend getting all of them (in order of GR, WG, then umbra being optional) but for the rjp the grim repeater does best for the boss
Lastly: the WG charge fires a slow bullet that goes through enemies doing multiple damage ticks, the alchemer charge breaks on impact then splits into weaker bullets, theyre not similar at all