Feel free to correct any calc errors...
Familiar Spit: Deal 100 damage every 0.6s with a magical projectile. Deals increased damage at 8, 9, and 10 stacks of Magic Mark on the enemy up to 1000 additional damage at 10 stacks. It's possible to apply 10 stacks every .8 seconds. That's 1375 DPS (spray damage not included) if you time it right.
Familiar Spray: 50/.08s=625 DPS - unlimited use.
Magic Barrier: Enemies that hit you while your Barrier is active receive 10 stacks of Magic Mark.
Theoretically Moji can achieve 2000 single target DPS.
A human will likely get a little lower than those numbers but it's still ridiculous and easy.
If you are in Moji's spray range - friggin HIDE.
If you shoot her while the barrier is up - friggin HIDE cause she can insta apply 1100 damage on you as you've been insta marked with 10 stacks. Distance isn't much of an issue here with 300 unit fully effective range - you've been marked she only needs to hit you once with her spit to get this damage. Some champions do area damage - Ash, etc. Good luck not hitting a nearby Moji if she puts up the Barrier.
Oh yeah, the spray can hit multiple enemies at the same time too.
You can all argue how she can be focused and killed blah blah blah but the damage output is excessive - period.
How about put the Spray on a resource system? Raising the skill floor a little.
PS: If a Barik turrent hits Moji's Barrier does that apply a mark on the turrent, Barik or does nothing?