Gigas can't damage these so if you are on old mobile then fill every slot with bullets then it will die, if you are on other platforms fill half the slots and it should be enough to kill it. The giga will also last around 6 minutes before it dies using one auto turret, so get some popcorn or start recording for that clip.

Note that if you are on old mobile it takes 80 metal and 720 gunpowder to craft 1200 advanced rifle bullets, but on other platforms it will take 600 metal and 5400 gunpowder.