The Concurrence in FF12 is an additional bonus you can get as a result of chaining together quickenings.
In order to go for this achievement you must first have 3 characters will a full 3 bars of mist ability, which can be easily gotten on the license boards. Once you have this the Concurrence have the following requirements, for 8 in total they are.
Inferno (3x Level 1 Quickening)
Cataclysm (6x Level 1 Quickening)
Torrent (2x Level 1 & 2x Level 2 Quickening)
Windburst (4x level 2 Quickening)
Whiteout (4x Level 3 quickening)
Ark Blast (2x Level 1 & 2x Level 2 & 2x Level 3 Quickening)
Luminescence (3x Level 1 & 3x Level 2 & 3x Level 3 Quickening)
Black Hole (4x Level 1 & 4x Level 2 & 4x Level 3 Quickening)
Now the game will always try to give you the best Concurrence you have chained, so if you are going for something like Windburst they you should stop once you know you have the requirements (even if you have more mist available to use).
As for tips on the harder ones, always start with the level 3s, and if you have an option to do a mist charge in the quickening you should ways take it. Unfortunately the last couple do depend on a bit of luck to get.