Fighting GUN-D4M again?

1John5vs7

Green Slime
Greetings all. I've been trying to find information on this but I must not be searching the right keywords or it isn't posted yet...

I have beaten the Beta so far in Story Mode (well, except the enraged Red Slime...I guess I suck since a lot of other people report they've beaten him...). Anyway, I want to craft items to get the best weapons and armors, but I see that many of the ingredients are drops off of bosses like GUN-D4M. I can't figure out how to fight him again, and he didn't drop anything the first time (other than Amalet, of course). Is there some way to fight him again short of hacking the game?
 
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