the title says pretty much everything
im looking for a manga that has humans fighting against monsters/aliens or else to survive.

with the hq and squad i mean that there are different headquaters all over the world or a country where the fighters train, go to school or live or whatever and wait for there orders or sth like that.
also it should be serious at least for live or dead situations

example that come to my mind:
D.gray-man
Freezing
Claymore though theres only one organisation/hq but anyway its awesome
Witch Hunter

Gate Keepers fits your request. They have different headquarters all over the world and they wait for orders to hunt their enemies down. It also has an anime (which I think is fun to watch).

[img]http://img844.imageshack.us/img844/7783/ariat.png[/img]

Witch Hunter comes to mind--humans against witches

alien 9 a squad of elementary girls with their personal alien vs incoming alien... just read it!
sadly the last scanlation release is almost a year ago..

thanks for replies.
other than Witch Hunter the other dont look very good
though i already read Witch Hunter i weill edit

maybe Outlanders (old art but i think it fit really well what you want, similar to macross in a certain way, except all the thing around the music, but more serious)

blue gender (anime)
macross frontier (anime)
macross 7 (anime)

best shonen manga couples :

shinichi X ran
natsu X erza
ippo X kumi
naruto X gaara

heroic age is an anime thats similar to this theme
and then you have the classic Robotech

No one has suggested Gantz?

Partially Soul Eater not so serious but it has its moments, also there are hq all over the world

i guess evangelion will do 😮

Quote from babybungo

No one has suggested Gantz? I second Gantz you really should try to read that 😎

[img]http://www.nodiatis.com/pub/18.jpg[/img]

Shingeki no Kyojin

aphorism
ga-rei
Kami to Sengoku Seitokai
world embryo

Quote from Aikanaro

Shingeki no Kyojin

I'm seconding this.

And although you may have already read this, Fullmetal Alchemist might fit.