Since you previously mentioned Svarog, here's a bit of number crunching:
Assuming 2.3 ability dmg modifier for Svarog nuke (probably the absolute minimum you'll ever have, should be higher in practice) and not enough damage to hit 300k/hit cap (which we prob will at that point, but lets go with that for simplicity) and your 180% assault. For simplicity i'll just take nuke on CD = 3 hits / 3 turns = 2.3 additional atk worth of damage each turn. So not completely disregarding the stackable atk buff, we already get a damage increase of 7.3/5 = 46%. That's more than Nike adds at turn 12. By then Svarog also adds 40% assault, resulting in 3.2/2.8 * 1.46 = 67% increase in damage.
But now once again, this assumes all other 4 are doing nothing but attacking, which would be ridiculous. You'd have to simulate a specific team setup, then compare it to another specific team setup, otherwise all that number crunching has no context at all. Even then, you can only calculate the exact CONSISTENT damage if you were using all skills on CD, which once again, doesn't say much. Pacing is so important in Kamihime (unless you only AAB) and Nike offers 0 options for that (besides the double full burst i guess). I love theorycrafting and number crunching, but it is only as good as your model and getting a precise model for every situation is impossible / infeasible. That's why playtesting is so much more important. Leave number crunching for things that are consistent and where the results actually have practical relevance, but don't rely on them stubbornly for more complex problems like this. Sure, do the math to get an idea of what it does/adds, but until you test it or see it in action, you'll never know how well a KH really fits into a team.
Once again, not saying Nike is useless, she definitely has her Niche uses, but putting a slowpoke like her into one of the fastest element teams for content that probably doesn't even last 10 turns most of the time might not the brightest idea.
Light's my main, so I'm confident a little dolphin like me will be able to manage. Thanks for the concern though.