This post is based of of personal experience.
(This only applies to those with 2000 cr after you craft the shadow item, and you must have completed the Sovereign Slime mission. If you don't have the crowns or met the mission requirement, you're screwed. Sorry.)
Ever had a shadow weapon that you wanted to heat up fast? But you used all of your mist and painstakingly earned CE to craft it? And you don't have enough cr to buy more energy on the market?
WELL FEAR NO MORE!
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Ok, no more advertisement.
It's really easy. Run around haven screaming "WTB 20 ce for ____ cr!" Replace the underlined segment with a price you deem to be fair. After a while, if this does not work, bump up your buy price slowly by hundreds. STOP bumping when you reach 2000 cr for 20 ce. If no one is interested, simply switch haven instances.
Once you obtain 20 ce, run the RJP (and DON'T DIE), until just before the elevator down to the Royal Jelly. You most likely cannot afford the trip down. That's ok, because you now have another 2000cr to spend on energy! Repeat the process as many times as needed to heat up your new shadow weapon.
Note: I said F2P in the subject because most P2P have plenty of CE to burn.
If everyone already knows how to do this, I'll graveyard it.
So your suggestion is to buy energy and play? How did you found out about that hidden secret?
Anyways, if you really need heat, tell a friend to help you by sacrificing his life. Reviving someone gives you 30% of their heat. (or play with noobs that die a lot)