@wildflower
I think there is a limit of how many stuns in a row you can land on the boss and after a certain count it will not get stunned for a full round. I can't tell for sure but i have noticed some things that are going on, compared not only total damage per round but also compared it to the max number of rounds. of course there is nothing i can say 100% because at the end there is a huge portion of luck on different levels (doing max individual damage, landing stuns and landing crits). however, i have noticed that if you prefer to have multiple stunners then make sure they are not hitting in sequence, best seems to spread them over subs. i am currently with 2 stunners and am doing significantly more damage than with 3 or 4. all i say you have to find a balance between trying to sun and doing damage. for example if a stunner like shayd is doing decent damage but is missing every 3rd round you have to take that into consideration over another stunner who is doing slightly less damage but never misses.
i have right now 2 setups with 2 stunners (aphro/cianna and aphro/shayd) each that i am testing and they seem to be the strongest and most reliable. i've had other forms using 3 and 4 stunners that did only half the damage.

also to make your lance do more damage give him lv 12 crit rune and aim for crit sancts (i know its luck, but removing certain sancts early and planning ahead helps) and the same for your other heroes. even your cianna can crit and your main (usually your first sacrifice is actually a decent DD) as well. grz on your "mooning", but its not as significant as you think because the critical damage is much more significant than a small raise in power.
good luck


ps: not for you, but to the other readers: often players experience a damage reduction after increasing shayds preciseness to 11 and 12. in my experience that is based on the fact that becasue she doesnt miss as much anymore it results into a higher frequency of stuns that lead to the main (usually at that stage without light spirit) running up the bosses morale and resulting into an early party wipe-out