DoomsdayG 10

The game tells you this: "Spell out FANTASTIC letters with the corresponding targets to activate Doctor Doom's ball-locking mechanism. There must be four balls locked via the Sign ramp to start this multiball mode."

Here's how you do it.
First you must hit all of the following targets:
FAN (these letters are on the right of Mr. Fantastic)
TAS (these letters are in front of Thing)
TIC (these letters are on the left of Human Torch)

Then you must lock four balls via the "sign" ramp leading the the 4 symbol with Doom on top.

This can be completed during another multiball mode; I personally did it during Thing's multiball mode on the 360 version.