After you completed it till you see the flow of time table thing destroy items blah blah blah. You finally understand everything and you are finally able to do side quests. So you can say that at that point you just completed the INTRO (and that point is at 47%.)
The last weapon fails 3 shots MAX long reload BUT it can kill in 1 shot. So it's only usefull for such big enemy but any other weapon can kill it fast enough for me.