What do you mean by "2 levels below"?

If you mean,, for instance, beating spider 12, but the team fails on spider 10, it's probably an affinity issue; champs getting weak hits and debuffs not landing because they're weak affinity

I use the same champions as I do not have superior champions- yet. only 4 Legedary and about 20 purple. Two of the Legendary and one of the Purple (Coldheart) are Level 6. The rest are 5s and 4s. I do not have a lot to choose from. This is my setup:

Opardin Clanfather L6 (Turn meter, A1 2 hits, Damage giver and healer)

Ultimate DK L5 (Healing shield and damage)

Alure L5 (Turn meter decrease and 3 hitter, wave assist)

Coldheart L6 (Turn meter, A1 4 hitter- A3 turn meter) 

Kael L5 (Damage, and A3 4 hitter)

Battling Fire Knight L15... Maybe 95% (if I control) win if on Auto 50%

Battling FK L14 100% (controlled by me) 80% Controlled by AI

Battling FK L13 100% (controlled by me) 100% controlled by AI

I am not keeping great stats on this and marking a ledger as I should...

This morning- there are no changes. It is the same setup up, unable to win on FK L15 at all..., and the same on 14. There are no changes, and I am wasting energy. I have to step back to 13 and manage the fight to ensure that I get bang for my buck.

This game is set up like a slot machine in Vegas- Hit, and you are lucky (like my Opardin)... not much else besides that. The game controllers (not being regulated by any gaming organization) can dial up or down the game to make it more difficult and frustrating, and they can set the algorithms to match the individual play style to draw more money out of your pocket! In the past... I just wanted to let you know that I reached back and paid. no more, no more, no more. I will lower my expectations and ever so slowly grind.

My initial comment would be RNG

which can greatly increase or decrease  your damage out  put  per level based on your chances to succeed

The largest of these variables  is CRIT RATE = which basically means  any chance your champions  on said team have to do extra damage

many other RNG variables come into play from other sources but this is probably the major one affecting your outcome

for fireknight 15 allure needs 100% crit rate to consistently decrease the bosses turn meter. Cold heart will need 150 accuracy to consistently decrease the bosses turn meter.

There is a giant difference between a rank 5 and rank 6 champion. I would suggest ranking your core dungeon team members all to 6 stars asap.

I would suggest posting a picture of your champion roster as your current team is not ideal for general dungeons. Coldheart and allure are however top tier vs fire knight but as above you need proper stats for them to work.

Have you set the AI so that in the first round against FK your champs are opening with a1s to get his shield down?

And that coldheart isnt using her a3 at the end of wave 2 so that it's cooled down for FK?

Thanks for the comments! I must farm better equipment and continue snail-crawling my champions to deal with FK.  All of which require silver and gems to make it to the top. This is an egg-chicken chicken-egg discussion regarding equipment and other means of outfitting them. I have Alure at 76% CR and Coldheart at 165 ACC.  The bottom line is that there is a bit of frustration as I was successful at L14 and 15 yesterday but am experiencing bad luck today.

its not bad luck, thats rng. Youre too early game to rng proof your teams.

I would suggest making a dragon farming team with all 60s and spend time there. In order to that you should be farming 12-3 brutal campaign for food to rank up your champions.

Your current team isn't really optimal at this time and not having kael 60 is going to make campaign farming harder.

I would suggest watching a youtube video on how to farm food. Then start working on getting a dragon 20 team together.

Side note if you have yet to book/mastery opardin clan father save your resources and bench him. He is a very mediocre champion who will see zero use in later stage dungeons.

Again I would suggest you take a screenshot of your champion roster anr post it here for a better idea on more optimal team composition.