Well, you gotta be unpredictable for one, that's a given. If they're using rockets, jump, it'll at least minimize the damage. Jump boots really help against rocket spammers. A special thing of Loadout's mechanics is that if you walk off a ledge, you sort of air-walk for like half a second. During this period you can still roll and jump. This can be used for some pretty neat jumps and shortcuts for either shaving off time when going somewhere or for escaping, I advise watching videos on these jumps and testing them yourself in game.
And of course, practice makes perfect.
Flak these days is just used to troll other launcher users. Hexabarrel Flak with Manual Det = norocket4u. Really helpful for Blitz, bonus points if you have a second launcher that actually deals damage.