HALO 2 GREAT JOURNEY JOHNSON GLITCH HELP

Hey guys I am trying to get the monopolized achievement as 343 does not admit that it’s badly designed, and I’m having a difficulty in spawning Johnsons. I play on coop, I get to the final boss with my spectre and johnson gets in, but when the second Johnson spawns the first one disappears and when I get the spectre to the area where the boss is, Johnson despawns again! Please help?