"Guardian Monster's" could be belonging to guardian monster, or Guardian Monster Is, but not the plural of Guardian Monster. Down with that apostrophe! Burn it! BUUUURN IIIIIT!
It's actually quite annoying, makes it look like some cheapass Japanese company made it, and couldn't be bothered to hire a translator, and hired someone's nephew to do it instead...
If you need an "'s" somewhere in it for the title screen, why not call it "Guardian's Monster" - and add a rudamentary story about an ultimate monster which has to be beaten in order to attain the rank of guardian or some such.
Ok, so that was the work of 20 seconds, and it still sounds odd, but you get an 's in there and it is gramatically correct.
Or "Monster's Guardian", of course...
Like Granny's Garden but with monsters and relying more on similar sounding words than any real connection for the second bit. So yeah.
It's actually quite annoying, makes it look like some cheapass Japanese company made it, and couldn't be bothered to hire a translator, and hired someone's nephew to do it instead...