1942: Joint Strike
12 Achievements
200
6-8h
Concerted Combat
You destroyed 30 enemies using a Joint Strike.
10
How to unlock the Concerted Combat achievement in 1942: Joint Strike - Definitive Guide
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TrueAchievements
TrueAchievements
on the P1 controller and go around 30 enemies with the joint strike, this is cumulative so this achievement doesn't have to be done in the same run even though you shouldn't have a problem, it took me awhile to figure this achievement out because there's no guides for this achievement, hope this solution helps!
RavenBlackthorn
Also really easily obtained even if you are playing with randoms on Xbox Live. Capcom ups the ante in terms of difficulty on co-op, so there are several spots throughout the game where killing 30 or more enemies is possible.
As far as the type of joint strike is concerned, the Clash Blast is probably your best bet. Since the Side Shot and Chain Lightning fire between the two planes, using these Joint Strike attacks require that you either circle the 30 planes or that they fly between you. The Clash Blast can be used independantly of the other plane and can cover a good portion of the screen.
The Clash Blast functions very similarly to that of the Big Bomb but requires that you press and hold 
to fire it. If you tap a small burst will fire on the crosshairs. If you hold longer the crosshairs get much bigger and can cover most of the playing field. Hold until you max out the size and let it go. In a busy area you should get this achievement very quickly.