If you think you can do it with just the autons and cubes, then by all means take the Lamplighter. It will take the Iocrym down pretty fast, especially if it's EMPd. However, if you think it may be tough with just the cubes, get the gem.
If you get the gem, I suggest running in, using a cube and deploying the autons. Once you've got the Iocrym scrambled and the autons out, use the gem and pull out a high-damage weapon that does thermo or some other high level damage type. The autons will EMP the Iocrym as its shields will be down, and you can tear up its weak armor with your weapon. Sounds like an easy win to me.
I remember the first time I fought the Iocrym, I cleared out an auton dealer to form a meat shield (or should that be auton shield) They too shome shots while I got close enough to kill the shields, then EMP it with the remaining M1/i autons.