So many things here...

1. Stop wasting time evaluating the performance of champs who aren't level 60. It's entirely pointless - ESPECIALLY when you're looking at something like survivability. Level 60s have something like triple the total stats, compared to 50s. Ontop of that, Level 60 unlocks the Warmaster/Giantslayer mastery, which is 100% essential for CB, along with the banner accessory, which is huge for accuracy and speed.
2. FB needs books *far* more than most champs do, because her A1 only lands poisons if there is a poison sensitivity. That is a *big* loss in damage.
3. I'm extremely doubtful about FB even being worth using. Her value in CB goes down *significantly* if you have no CA or AA champs on the team. Four poisons every three turns is extremely underwhelming, especially when you have BEK, who not only can place just about as many poisons, but also provides secondary healing, a great aura, and, more importantly, a very strong passive.

Well, I want to evaluate as best as I can before investing a lot in maxing a champion.

In CB I have: Kyoku, BeK, Kael, Ronda and one more spot. With the 2nd Kael (50) sometimes I do 12M+ in brutal but I want to optimize this spot (a spot that can also be used in Dragon). Who would you use instead?

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Your team isn't tuned, so I wouldn't really concern myself much at all with the exact comp. Just use five 60s and focus on other stuff. When you're ready, build an actual tuned team. And I expect FB will not be on that team, so I don't think you should spend any resources just to bring her to 60 for this.

I agree with Krama. Just put Genzin in the last spot of your team. His A1, looking at multiplier and atk stat, is super strong, and he brings decrease def with his A2, what your team is lacking right now, and, most important, he is lvl 60 allready. Don't invest in champs that will be used only for a short period of time, that's just an ineffective waste of resources. Especially don't bring a second Kael to lvl 60, I don't understand why so many players make this mistake and build multiple Kaels.

Instead, build champs that are usefull in other content of the game. Armiger for FK and Spider comes in mind, Achak is probably worth lvl 60 as well.

Two champs to build allready, and you will get more from log-in rewards (Scyl at day 180) and maybe shards.

If you're 2 keying Brutal CB with your current team then I'd stick with what you're using. If you want a poison champion then Anax is 10,000 times better than FB. He can put out Lego level damage when geared properly. 

I would suggest spending some time focusing on gear as well. I'm just looking at Ronda since I can see her gear. I'm not sure how your other champions are geared but I'm assuming about the same since she is on your CB team.

To excel in earlier CB you need speed and crit rate. Try to get to 100% crit then put on as much speed as you can. Relentless gear is amazing as long as you can get to the stats you need. With no speed and 40% crit rate it's really not doing you a lot of good.

Don't focus on gear sets. Focus on the pieces that get you the stats you need. Ronda with 100% crit and more speed will easily out damage the bonus from the Relentless set you have her in.

Yes, Skadi, one of my pending tasks is to improve Genzin, he's clearly underperforming because he has no good gear, no ascensions, no books ... obviously Kyoku and BeK are first on the list but at least I must get him better gear and 6* ascension.

Armiger ... I always forget about him ... I have Maneater to remove the spider's TM but I guess that having 2 is better than 1. 

Well, if I can 4-key NM with a little effort I think it's worth (getting access to a chest with the chance of a sacred shard and more ...)

What I see very interesting with FB is:

- Poison in A1

- The Poison Sensibility. When booked, the boss can be 2/3 of time with poison sensibility, so he takes much more damage, not only from FB's poisons but from Kael's, BeK's ...

- She's rare: much easier to book, ascend etc.

Thanks a lot for the advice on Ronda, I'll try to re-gear her these days.

There's a world of difference between Brutal and NM. If you're only able to 2-key Brutal I think it's unlikely that any team we build you can 4-key Nightmare.

But, more importantly than that, even if we did build you a team, it still wouldn't be worth sacrificing all that gear and energy into a team that almost certainly will get remade later.

And, lastly, while there's a small chance FB may improve your damage for now, it won't be worth it in the long term because there's very little chance she'll remain usable anywhere at all.

Will leveling FB to 60 and using her on your CB team improve your team's damage? Sure, I believe it will for all the reasons you posted above. No one is saying she is a bad champion at your stage of the game. What you have to understand is that the people who give advise here are speaking from a long term perspective. In a few months, when you've progressed your account, pulled better champions, and gotten better gear... FB will be just another lvl 60 Rare that sits in your vault until you need her for a Doom Tower secret room or Faction Wars.

As kramaswamy.kr said above, the difficulty of CB increases exponentially as you progress from level to level. For example, I can currently 1 key Nightmare daily doing +/-50M damage with a killable team. That same team averages around half that damage (+/-27M) on Ultra Nightmare. 

What I found from my CB experience with Poison damage is that it falls off rather quickly as your main damage dealing skill once you start getting into the higher CB tiers. My original CB team through Brutal was Urogrim, Kael, FB, Fayne & Godseeker Aniri. This team served me well but @ Brutal I hit a wall and was unable to do better than an occasional 2 but normally 3 key. At that point I had to re-evaluate my team set-up and damage profile.  

CB has a damage cap for single skills. This is mainly directed at skills that do damage based on enemy max HP. Because clan boss has significantly higher HP than any other enemy in the game, poisons (2/5% of enemy HP), HP Burn (3% of enemy HP), war master mastery procs (4% of boss HP) or Giant Slayer procs (3% of boss HP) would be way overpowered otherwise. 

At the end of the day it's your account & your CB team. If you want to build FB because you like her, like her kit, and you feel like you will enjoy playing with her then go for it.

Mmmm, I didn't know about the type of damage with CB. I appreciate a lot the help given here, that's why I keep asking :)

There should be this weekend a 2x with sacred shards, I'll wait and see if I get someone interesting (healer/reviver, TM/speed champ.) and re-evaluate.

By the way, I've changed some gear now that it's free, and I'm trying Bellower with a freeze set (stun gear not good enough), and so far it's quite dissapointing, just one freeze in like 3 turns with mobs waves (1/15, very far from 20%). He has 193 Acc and it's Dragon 17/18. What am I missing?

Freeze is reactive. It triggers when *you* are attacked, not when you attack them. If your goal is to help CC the waves, you'll want Stun or Sleep, not Freeze.

Also, set bonuses like Freeze do not require accuracy.

Thank Kramas, I guess I'll change gear again ...

i

Well, I think that the "little effort" was worth it: a daily Ultimate Chest.

Well done. :)

I've been using FB on my CB team for over 3 months and she's definitely helped a lot with progression there. FB is also useful in early game Dragon's Lair - not much good against waves, but can be great against the dragon itself.

Yes, from my experience, jumping to the next level in CB means approx. 2/3 of the damage. When I saw that I was making 15M in brutal (FB was still 5*), I decided to go for it and it definitely paid off. She's not making tons of damage herself but I think she helps a lot the other poisoners to do more damage. There are still some issues like dealing with the not-void stage of the boss, but currently in my clan not many people (as you can see) fight CB so ...

I haven't tried her in Dragon yet but I will. I managed to beat level 20 with some changes made with the free change of gear (don't stop doing this from time to time Plarium, please) so I don't want to change anything in the team right now.

Now the goal is getting better at Spider, where I'm still struggling with level 13,14.