To easily gain Meseta, after you reach a high enough level, go the the Synthesize shop and purchase the Muktrand Board. Go to your room and insert the Board to your Mag. For "???", use the Photon. This will increase your chances of getting the Muktrand. Take the item out. It will almost always be the Muktrand which can be sold for 392,000 Meseta.
There are a couple Slayer titles thatare unlocked by defeating the enemy of the same name. You can defeat De Rol Le to become the De Rol Le Slayer. You can defeat Mother Brain to become the Mother Brain Slayer, and you can defeat Slterazgohg to become the Slterazgohg Slayer.
To easily gain Meseta, go to the Flyer base on Planet Parum. Look through every item until you find a gun that is basically a monster on a rope. It shoots comets but has low accuracy. It can be sold for 32,000 Meseta.