actually ai.obj is compiled from ai.nasc (witch is source)
you compile ai.nasc trought l2npc.exe (witch is npc server(is like "virtual machine"))
you can use already decompiled an manually fixed version of GF ai.obj(witch is decompiled by sauron decompiler for c4(and manualy fixed to work like a charm..))
you need just to edit and compile the ai.nasc, you dont need to decompile it again...
to run l2npc.exe
just launch you cmd -> then switch to your l2npc dir (via chdir command) and then type : l2npc.exe -c -e