Is there an editor for PS2 (like Aridia for PS3) were you can edit maps, enemies and that sort of things? I wonder how the hordes of nei patch was made...
There are a couple save state editors but no ROM editors that I'm aware of. The author of Hordes of Nei (which is a PS1 patch) did it manually with a hex editor.
Indeed i was talking about Numan Revolution / Evil Overload (and not Hordes of Nei) altough they are hacked by the same person (Hacked by The Komrade). So the only way is to hex-hack the game