You arsenal lacks some shadow damage, consider switching your mighty blast bomb with a dark matter bomb and your cousin should consider getting a night hawk after you and him managed to defeat JK a few more times, and switch out the blaster with the night hawk. This way you might find it easier in RJP since slimes are weak to shadow. For your drakon perk, you should use either liquidfier(dmg bonus vs jelly) or mighty boom (increases bomb dmg overall)

If you and your cousin are always playing as a team, then what you is lacking in is some swords. Consider getting a slient night blade, it is currently the best slime exterinating weapon in the game and currently the most OP weapon in the game, one of you should get the sword and use its charge attack to wipe out large group of jellies and Gremlins alike.

If you are the one with night blade, you can combine it with your freeze bomb, use it to freeze the enemy so you have enough time to charge your sword. The charge attack releases a straight line of purple blast that deals massive damage.

If your cousin is the one with night blade, tell him/her to not getting the nighthawk and make the gun into a silversix instead, the gun will come usefull fighting lumbers and silkwings as well as your end game.

"But I wanna be a pure bomber D:"

Don't hang yourself dead on just one type of weapon, be flexible, this game was never designed to be beaten with just one type of weapon, srsly, you can't fight the final boss with bombs along, well you can, but it is not the optimum way to do it.

Freeze can be very useful in some situations, obviously. But in other situations it is actually counterproductive, because it hinders your ability to kite, knock back, shield bump, etc.

In my opinion, a freeze bomb has little value in RJP. Here are some good bombs for RJP:
* Dark Retribution: This is awesome, but it requires Operation Crimson Hammer.
* Ash of Agni: Fire destroys all of the mini jellies quickly, and hence hinders RJ's healing.
* Voltaic Tempest: When the monsters are tightly clumped, shock can actually do more damage than fire.
* Venom Veiler: Poison is an underrated status in general. In RJP, it hinders RJ's healing.
And the shadow shard bombs and vortex bombs are probably useful too.

Yes, Nightblade/Acheron is an awesome weapon, but I encourage you to try to stay pure bomber for a while. Hybrids are flexible; giving up this flexibility makes the game more interesting.

2* mist bombs are apparently garbage, so Lightning Capacitor would be a good idea, even though i would recommend AoA more.
Dark Retribution cannot be understated enough.
In a team which plays well together, Graviton Charge works nicely too. Blast bombs don't play well in a team, so avoid using it in parties.
Read this , it's very informative. Watch the videos especially.

5* bombs are pretty much out of reach right now with the lack of radiants, especially for bombs that use the explosion for damage rather than a status--your Master Blast bomb does more damage at 4* heat level 10 than it would at 5* until about level 7, and the importance of getting max CTR. Blast bombs do need damage bonuses to be effective, but with a teammate you know it can still be useful in a party. You friend is a gunner after all, so having an area kept clear of enemies by the knockback of a blast bomb would be more helpful than it would for a guy using mostly swords.

For mist bombs, you can go pretty far with 3* mist bombs, back in the day before the sprites/energy update I used mostly a 3* fire mist bomb to solo an end of tier 2 arena without bomber gear. 5* mist bombs are more possible since 72 radiants would get one to level 5, so you could use your normal gear for getting max CTR and still have ultra CTR on it, but have increased radius and mist time of the bomb.

One last thing, you get the 4* Plasma Capacitator recipe free from the rank mission 8-2 The Great Escape, so you don't need to buy it. I also use bombs a lot, if you would like to do some runs together or help with rank missions send me a friend request to my main, Hareth.

Trats-Romra, Stratum N is designed for (N-1)-star items. So, for example, Stratum 4 (where RJP is) is designed for 3-star items. If you are experienced, then you can beat RJP using 2-star items. But it will definitely be easier with 3-star items.

By the way, when giving advice I always name the 5-star versions of weapons, such as Voltaic Tempest. But I'm not saying that you need the 5-star version for RJP. I really just mean "whatever the appropriate star-level is, of the weapon that eventually becomes Voltaic Tempest". Halandin is right that heating 5-star items is very difficult nowadays. Don't be surprised if you hang out at 4-star heat-level-10 for a while, saving up radiant fire crystals.

Status inflicted by 2* weapons is just as strong as status inflicted by 3* weapons. However, the increase in radius is still helpful.

For RJP, I recommend any of these bombs:
Blast Bomb line
Fiery Vaporizer line
Rock Salt Bomb line
Static Capacitor line
Toxic Vaporizer line
Graviton Charge line

In no particular order. Any of those bombs, should you learn how to use them, will let you very easily deal with most of RJP. If you're using a vaporizer or a vortex, try to keep all the slimes on one side of you and backpedal slowly, baiting them into your bombs. If you're using the damage bombs; pick a spot, plant a bomb there, run outside, and run back inside with another bomb just as your previous bomb goes off.

The wiki has a bomber guide with tips, guidance and videos.

I would not recommend freezing vaporizer, as frozsen slimes can still simply use their spike attacks, and those are no fun to deal with.

Put the slime damage or CTR perk on your seraphinx and stuff all your points into heart attack. The shield is useful, but last time I checked it's broken and not affected by stat points until you get the ultimate.

For general tactics, your role is to deal with the masses of slimes while your gunner cousin should target the silkwings and constructs, in that order.

Slimes and constructs are both weak to fire, so he should rely on the firotech alchemer for the majority of his battles, using the blaster to deal with the silkwing. He might also want to consider getting a swiftstrike buckler if he feels comfortable trading shield strength for gunning speed. He should be using slime or construct damage perks.

JK is a boss made to test the might of a person (compare to RT which relies on one's ability to time missles). Shiver, is a good bomb but not very useful unless your playing support, blast's also good if you cousin doesn't mind stuff flying left, right and center.

And from personaly experience your going to grind JK till you can get a 4* gear up and running, so i advice the salt bomb, not very poweful but not as hindering as blast bomb (as it progresses to the ranks) plus it works as a mini VT.

I'd give you more tips on Tier 3, but you'd best focus on the mission at hand. Btw, you will come back for more question if needed :)