If you want to run Coil unsynced, do it on Tuesday after the reset. That's when everyone's Wondrous Tails books reset, and folks are looking for ways to fill it in quickly. Even T5/T9/T13 fights fill relatively quickly on Tuesdays! Just put up a party finder with the message "Unsync for WT", and your party should fill in under ten minutes, no matter which Coil you're doing.

As you noted, though, even unsynced some Coil fights will nail you with instant death mechanics. The fights most notable for this are T5 and T9, but you can also run into trouble in T8 and T11. For those fights, study up on the fight ahead of time so you know what to expect. Chances are good that most or all of your pick-up members will be familiar with the fights, so as long as you do your part, they will do theirs.