Yeah, the 'cross' that you're talking about is there in order to install and remove that fitting.
If it's gone... well, it's going to be tough, and scary to remove it.
Risk of damaging fiberglass = moderate to high
The easiest option would be to go below, and cut the pipe going to the fitting that your 'strainer' screws into, and see if you can spin the pipe below enough to be able to get it loose. (leave enough room on both sides for a coupler to put it back together with)
DM's suggestion will work, but those things are usually pretty thick, and you might have a hard time bending it without gouging the fiberglass.
You can try cutting the metal out of the lower fitting using a hacksaw or a sawzall, but you'll probably ruin the lower fitting (you should be able to buy a new one) and if you're not careful, you could hit the fiberglass and really mess it up.
Those are my ideas. Good luck.
Edit : Unless they make an EZ-out that size.... you might get it out with that. That would actually be the easiest.