Was bullshit!

While Light Nike is stronger than on-element Hime, the actual number is lower. Around 14% is more correct - still a substantial upgrade over any on-element Hime, but not quite P2W Eidolon level.

So, let's go over that 14%.
Light Nike brings 180% Assault to 300% Assault, so 4 (100% base + Light Nike's buff + weapons) / 2.8 (100% base + weapons) = ~1.428
Because we can disregard Light Nike doing ANY damage off-element, you have 4 Hime (well, 3 Hime + Soul) dealing damage. So 4 Hime * 1.428 = 5.714 damage output.
5.714 / 5 (which would be the normal amount of damage you'd get with five on-element Hime) = 1.142
Thus, ~14% increase. Of course, we have to remember that at the very earliest, this applies on turn 11. Content doesn't take you that long? Don't bring Light Nike.

Of course, this number can be mathematically challenged fairly easily. For example, this calculation entirely ignores damage caps (pre-Light Nike 700k burst hitters would hit damage cap with Light Nike) or damage skills used by the potential fifth party member. Please do challenge it - I'd love to go through the subject more thoroughly.
Okay if we ignore the fact that her buff takes 11 turns to kick in, that's 14% damage increase over on-element Hime. Nothing seems wrong, except the fact that it assumes the Hime replaced by Light Nike will just auto-attack without any skill or passive.
Let's take example, with Dark team, since Dark is probably the weakest in damage, and slowest burst ATM: A ideal team will probably be: Hero+Satan/AmonU/Thanatos/Samael+Pluto+1 strong attacker (Let's take Chernobog in this case)+Wild card.

If that wild card is L.Nike: 14% increase.
If it's anything else, which ideally will be a second attacker, we have: Berith (Counter, a faster and stronger self stackable atk buff, berserk), Susanoo(again faster, stronger stack, strong nuke), Kali (Pride-based atk and DATA buff), Agalia (Strong crit buff). All of these easily provide more than a mere 14%, u dont need to be a math genius to see this, people have prove this by actually using them.
Same case for other element, an on-element will always bring more damage than Nike can (with maybe extra ultility as well), so even if we ignore the fact that off-element Nike can hardly last more than 3 turns in endgame content, the ided of bringing her for the extra damage is just retarded.
Now back to Light
Because when it comes to Light teams, nothing even comes close to Light Nike's potential.
This, this will trigger any Light user out there; in fact, it already triggered the Rank-90-in-Tower-Bear up there, he already confirmed that AW Mike, AW Metatron, Takeminakata, Frey, SSR Arty,.. would yield better result, but apparently you, who didnt even have any real experience with most of them, disagree for whatever reason.
Even just by looking at them on paper, this is what i see:
- Light Nike will bring either roughly 30% damage boost, or double FB at turn every 10 turns, coz having both is just unrealistic in endgame scenario.
- 30% damage is something easily achievable with any Meta-attacker coz they can easily do 2 to 3 times damage of a standard defender, debuffer,...
If we take the Double FB approach, so it is DFB every 11 turns, or 1 FB every 4-5 turns+Elemental dmg buff, dmg cut+Nuke+DA with AW Mike. It's clearly as day which one is generally better.