I am free to play

Kael first, assuming he doesn't have masteries already. After that, the priority for this will usually be your Clan Boss champions - so Frozen Banshee for instance. But not sure why you're building a second FB, that won't do you any good. You'd better focus on useful champs like Warmaiden, Spirithost, Coffin Smasher who you're currently neglecting. 

I would advise to use your Gems to upgrade the Gem Mine to maximum as a priority, if you plan to play for a while. It will pay back in 100 days. From then on, it is pure profit.

Contrary to the popular "obsession" with Kael, I would rather advise getting Athel to *6/60, buying the Masteries for her, then concentrate on getting Frozen Banshee to *6/60.

My reasoning is, that using Kael and Frozen Banshee on Clan Boss at once is a bad idea, because Kael will waste debuff slots on the Clan Boss with his weak poison. With a F2P's limited resources, I would choose to invest in Athel, who will be a great help with Fireknight (3-hit A1) and Spider (A2 AoE hits like a truck). She also has Weaken. She is a great and reasonably fast Campaign farmer, I am still using her for that at account level 69.

Warmaiden would then be another good champion to invest in for AoE Decrease Defence. As a farmable, she is easy to book.

Spirithost will help you in the Arena,and is a good cleanser, but I would rather get High Khatun to *5/50 first to get a good Speed leader for the Arena. As a farmable, she is easy to book. She will help you in Minotaur and other places with her cleansing.

Another gem you have is Soulbond Bowyer. She is awesome with lots of Speed and Accuracy, and her 100% chance of turn meter reset on fully booked A3 is fantastic, as his her A2 with ignore 75% of enemy Defence. Definitely worth booking, but she needs whopping 16 Rare books.

Coffin Smasher is apparently great for Clan Boss, although I have never used him there. He is another candidate for investing your resources, as he can decrease Attack and place HP Burn. Two crucial skills for Clan Boss. He needs 14 Rare books, though.

You also have Luria. A bit tricky to book as an Epic, but she will help you a lot, especially in the Arena and Ice Golem, with her AoE which blocks revives. Her A3 also removes all buffs on a single target.

I would definitely recommend putting together some Tag Arena teams as soon as possible, and starting to collect fragments for Drexthar Bloodtwin. It will take a while, but he is a free Legendary (my first lucky pull) and has been the team leader of my dungeon teams since getting to *6/60. He is a fantastic tank, he has HP Burn on a 3-hit A1 (great for Fireknight and Clan Boss) and passively on getting hit, he is one of the few champions that can benefit from both Attack and Defence for his offensive skills, and his +70 Accurycy Aura is plain awesome.

Join a good clan, spend your Clan Boss keys every day. You will get a good source of nice drops, including books and shards.

For one team on tag I would suggest you build up that shatter bones he is really useful if you dont just auto it. He needs high accuracy and the speed to go first so put him with high katune or anyone else who gives high speed boost. Use his a3 if I remember correctly its called wither it reduces their turn meter then hit with a high aoe and a buff blocking from some one like Erinyes. And then fight like normal for the win.