Pinball FX2

This can be very confusing for several reasons. First, you must play as Captain America or this won't be available.
What you must do is knock down 1 target, then hit a specific ramp. Sounds easy enough right? Well, the ramp is invisible.
After hitting the first target, shoot towards the lower left sink hole. If done right the ball will swing around the table on a lit up ramp. You have 15 seconds to hit this ramp. If you don't, the target will come back up, just like it did for me the first time. Once done correctly, this target will stay down til this particular sequence is complete. It's possible to hit it into the sink hole and miss the ramp so keep trying if you don't get the lit up ramp part.
BTW, don't hit another target til you complete the first target and ramp. It's possible to knock down all 3 targets and get into the prison ( that's what this part of the game is called ), but that won't get you the achievement.
Next hit the second target. Now the invisible ramp is on the right.
Complete that part and then hit the final target. No more invisible ramps, just hit the ball into the prison for the achievement.
This completes this sequence and starts another. After which it can start again. Good luck.

Toward the back center of the table there is a blue swirling portal entrance into the Negative Zone to reach the superhuman prison The Raft. Entering this portal is your goal. It's a little tricky as it is a small area to squeeze back in to. I found it easiest off of the right flipper, a little past midway down. (Although it is very possible to hit it off the left)

Once the table starts (after the Stamford multiball, skillshot, etc) Iron Man will place three barriers or 'guards' in front of it. You must clear these three targets in order to make it to the portal. After hitting a guard it will remain down for several seconds.

HITTING THE PORTAL BEFORE THE GUARDS DROP WILL NOT GET YOU THE ACHIEVEMENT OR START THE RAFT MODE!

During this time a special ramp will light up. Hitting this ramp will lock the guard down for the remainder of the game, not just the ball. Note that these ramps are not visible until you hit them. You will notice their location is marked by a flashing signal below the target. Once you hit them, the ball moves very quickly on a track made of light. If you miss, the guard will pop back up and you must attempt it again.

After hitting the first guard the ramp on the far left lower side of the table, above the S.H.I.E.L.D. hole will light. Hit this to lock the guard down.

(At this point you should get the gist of it.)

After hitting the second guard the next ramp is on the exact opposite side above the other hole. Hit it to lock the next guard.

Finally, take down the third guard and hit the same shot again and into the portal before the guard pops back up.

Once your ball finishes swirling into the portal... BAM! Achievement pops.

It's really not too difficult. Managed to get it fairly quickly after downloading the table. Best of luck and happy hunting.