Giant Shivan during tutorial
Probably not the best idea since this would be one of the first experiences that any new player has to the game. If they were to have trouble figuring out the game for some reason (computer issues, a relatively young player not familiar, relatively older player not familiar, etc) it could come off as extremely frustrating to them to get stuck in the second 'zone' of the game forever. Better to get them through that part, and into Atlas where more players can help and GMs are probably more likely to find them easily, not to mention there's less 'pressure' from a giant orange jelly fish looking thing.
Yes, I know the thing can't kill you, but a new player probably doesn't know that. Trapping them there with a 30 foot monster, air raids, and with a city crumbling around them is a less than ideal place to get help starting out.
Besides, what does it hurt?
If players just stand off to one side and refuse to help defeat the Giant Shivan, how about not advancing them to the ambulance?