fastest way to level?

somehow, Yesterday this guy said he will sell me a venom veiler once he level's up the 4* bomb to 10. however, this only took 1 day, and the trade was quick.
This got me wondering: how do people level up their gears to 10 so fast?
I mean, I barely see any increasing in heat with my current stuff, so it got me curious.
If you know any tips, do tell.
Pretty much what sirthinh said. Jelly runs are fantastic for leveling, especially the last two levels before the king. Make sure you level the gear you use most first, so when you're trying to level another piece it won't take as long, with the added bonus of being able to wear your favorite gear without worrying about heat splits.
Go to jelly castle. Bring some friends. Just before the final mob dies, let it kill your friends a couple times. Revive them for their heat.
Except that you should wear every piece of equipment that requires heat. The total amount of heat acquired on a level is applied to each piece.
One piece at a time is not economical.
@GrimmSweeper Thought this was about the fastest way to level upto 10?
Otherwise, I used to bring in an alt character @ depth 16 and just let it die and res it a bunch before JK like Stardrinker said if I was on a rush job.
And it is. You missed the point I was trying to make there. Heat distributed to items does not change whether you have one piece of equipment that is still <10 or several.
Can you cite that? Because the common belief is that it is split between each item.
I thought it was common knowledge that it wasn't.
It's split.
I'm leveling five pieces of gear at the moment and I can tell you it's very noticeable.
Yeah, I did the same thing when I got all my 4* gear, and it took a LONG time to get it all to lvl 10.
When you hit 10 with one piece, you can see the others visibly accelerate.
Leveling only one item at a time, dont die, res everyone, and constant jelly runs. Should be able to get 4* weapon to level 10 in a day no problem.