I haven't really read Gensokyo Tenshi or Monster Hunter Gensokyo. That said, I'm going to now. This was really good.

Love it, plot is interesting.

...and... First!

Wow i got interested on the plot, of course MariAli too, but The AU really got my attention, it seems to be rather interesting

A few things to nitpick over, but most glaringly, the honorifics. They shouldn't be capitalized, as they're a suffix rather than a whole 'nother word. Also, I'm pretty sure Acha won't mind the extra publicity for MHG. The stories are similar, but there's also a lot of key differences. Third, Alice's lack of a rifle. More a personal preference than anything else, but most urban use sniper rifles are actually fairly lightweight and probably within Alice's capabilities. She doesn't need a Barrett to operate as a sniper in city limits, don't you agree? Lastly, and this is more a factual thing than a personal one, the assault rifles. Unnamed, but in urban settings, it'd make more sense to use a tactical SMG like the P-90 since most assault rifles have long barrels that can be grabbed from round a corner.

On a parting note, bug Acha about weapons if you get the time. He knows quite a bit and can help flesh out and stabilize your story. Just tell him that you're bugging him on my recommendation so he doesn't mistake it as something else.