There are lots of data sets and guides for farming. Some of them are linked at my FAQ:
https://wiki.spiralknights.com/User:Jdavis/FAQs#How_do_I_earn_crowns_qu…
I'm sorry that you're finding the Royal Jelly difficult. Doing earlier missions won't help that much (although going through the rank missions is valuable in general). Have you tried standing in Haven and asking for help on the Sovereign Slime mission? Also, are you doing the mission on Normal difficulty? That's the easiest level.
For tactics against the Royal Jelly, you can also view videos on YouTube. You want to focus your attacks on the Royal Jelly, pretty much ignoring everything else. Also, use vials and barriers to keep him on fire, poisoned, shocked, etc. Status helps a lot.