I have encountered that bug a few times already. On my Ipad, no streaming options will come up occasionally and just funnel to the youtube playback. This wouldn't be that big of an issue if the youtube quality wasn't garbage. Very poor choice.
I got the assault rifle with my game, and yeah, it came with 6stars, and I felt the same as you. I never used it once during my playthrough, though I will start trying it out in post game and if I go back through the game on the hardest setting (this seems doubtful).
I used Fang/Hope/Lightning, stacked +STR items on Fang and Lightning, started the battle with Haste on Fang and called Bahamut. Have Fang go SAB and debuff the hell out of the turtle, then when the summon's meter is almost empty go into Gestalt mode. You should end up taking about 1mil from the turtle from the summon.
Then return to battle in SAB/SYN/RAV and buff up everybody and debuff the turtle some more. Switch to COM/RAV/RAV (or RAV/RAV/RAV if your Fang has rav skills) and stagger him, then switch to Com/Rav/Rav to hopefully finish him off before he gets back up and destroys you. This sometimes works, and sometimes he gets back up and kills you. Still, this is a little less cheap than the Death method.