Rogue Galaxy Quarry: The Shapeless Assassin
In order to reach Megalomantis you'll want to teleport to the area "Near the Pit" in the Cancer King's tunnels. From the save point I told you to teleport to you'll want to head to the location on the map seen in the screenshot. It's only a very short run northwest. Once here you'll want to use a Smoked Rainbow Newt in order to initiate the boss fight.
Smoked Rainbow Newts can be obtained at the Insectron store in Zerard if you're out of them. Usually they have a decent supply in stock. Closest save to port to the Insectron building on Zerard is the East Side.
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If you are missing the item required to summon this boss grind some battles and cross your fingers or go treasure chest hunting on a planet you've 100% the map on. In Rogue Galaxy once you pass a certain chapter the loot table of items that can drop for you ingame is updated.
What I mean is, every item has a chance of dropping off every creature after you've passed that chapter that allows that item to drop. At least this is how it works in my experience. Therefore, if you're missing any items to summon any of the quarry bosses all you need to do is grind a little until you get that item to drop for you. Good luck!