Frozen Banshee and Royal Guard are good choices for the next lvl 60 champs. 

May I ask how long you are in the game? I don't see High Khatun, who is the lvl 30 log-in reward. I hope you didn't use her as food. If you are below day 30, you made a great progress to your account with two usefull lvl 60 champs (Kael and Apo) allready. Go on like that!

Thanks for your advice! Would you rather choose banshee or royal guard? Banshee seems better for poison, but the speed decrease royal guard offers seems pretty insane as well

For my progress: I am currently on day 16, I wouldn't use high khatun as food :) I bought the daily gem pack & a void shard pack, which included a 5* chicken iirc. That boosted my progress immensely. Also I've been doing nothing but grinding food for like 14 days, I started advancing the progression missions yesterday, which is why I entered the dungeons in the first place (I got the advice to stay away from dungeon until I have 3 to 5 6* Champs). 

I would take Frozen Banshee to 60 first and try to progress in dungeons a little bit more to get better gear allready. Royal Guard needs some good gear to use his full potential, and maybe you don't want to spread your best gear among to much different damage dealers (Kael, Ninja and Royal Guard) at the start.

You could even wait with Royal Guard and make Grizzled Jarl lvl 60 after Frozen Banshee. Every champ in your CB-team should be lvl 60 and have either Warmaster or Giantslayer masterie. Even support champs with low damage can deal millions of dam at the Clanboss with that masteries.

dragon progression before CB, rewards are sh!t until NM/UNM.  Jarl (will actually help in CB and dragon more than other 2) before FB and RG.

Agree with Trips here - yes, FB is one of the best poisoners on CB and on the dragon too, but the increased survivability from a level 60 Jarl will probably make more difference in your ability to reach higher dragon stages / last more rounds on the CB. FB and RG are both worth 6* after that. 

For Spider, keep an eye out for the uncommon Armiger - he's an uncommon so easy to get, and he can help a lot there in the higher stages where it becomes all about controlling the big spider's turn meter. Your void rare Paragon may also help there, as well as in a few other places, because of his unique ability to make himself permanently unkillable (when booked). But *don't* level Paragon up (you want him to be targeted while unkillable, and the lower his HP, the more likely he is to get targeted, so leveling him up may actually make him less effective).