Latias: A massive problem, in the same vein as Hydreigon but substantially weaker. Calm mind versions are a huge bitch if Chomp gone, because it has no problem setting up on Magnezone. I'm pretty much required to keep chomp when dealing with Latias or I lose. I can somewhat check it with a combination of Chomp and Mamoswine but even thats not enough and its part of the reason why I sometimes hate running HP fire on Latios.
Yep, unless your Politoed is a fucking cheat and crits or freezes opposing Latias's, you are in for a tough time. Sub/CM would pretty much guarantee that something on your team dies. Not too much you can do without completely changing your team, but I would suggest one simple little change to improve your situation slightly - Change Dragon Claw to Dual Chop on Garchomp. With this, you can revenge Latias with Garchomp Sub or no (at least put it to the position where it won't sweep any more).