This Mechat stage can be found after you have access to your own Mechat, fly to the southern-most yellow marker on your world map. You should find a large flying purple base called Upper Mecha Base, as you get close you will see the A button in the lower left corner, push A and you will begin the stage. You need to destroy every gun in both parts of the base, you start low down where there are lots of machine guns/cannons and missile launchers. You should take out all but one of the missle launchers using your own missiles and then use the machine gun to take out everything else. Once you destroy the last missile launcher you move onto the next section.
For this part you want to take out the large spherical ring laser guns as quickly as possible to avoid damage, once they are all down destroy the missile launchers that were behind them. There are also a large number of machine guns near the centre of the base that need to be destoryed and four larger machine guns around the perimeter. You have plenty of time for this section as it only ends once you destroy the large spire at the centre, just take your time and be sure every gun is destroyed.