Sunset Overdrive

After playing through a full game of Chaos Mode, you end up with a slot machine.
The slot machine has 5 slots; Each slot is unlocked depending on the score you receive at the end of the full game.
Eg, first one is free, second 20k, third 200k, fourth 1million, fifth 45million scores.
(these were example numbers - the numbers will vary depending on which map you are playing as well.)

Once it calculated how many slots you have earnt, it spins them all.
There is a small chance of gaining an amp upgrade, most of the time it is dollars or overdrive.

This is very much a luck based achievement but the better you/your group does in the game the more spins your receive.

I have now got this achievement, and can confirm I received it from upgrade in 3 spin slot. I received Bounce-Splosion Lvl 2) - which is an Epic Amp)
Then got another amp 2 games later in 4th slot.(Bear Force One Lvl 2).
Can also confirm second slot gave amp.
Still have not seen free amp in first slot.

Fiatim confirmed that the prizes from slots are already calculated at the end of the match, so the achievement will pop just before you get to the slots screen.

An extra tip to increase the likelihood of getting an amp upgrade is to play chaos squad in the first district (Old Factory).

This district is the easiest and quickest one to get through. Equip a powerful weapon, ideally a level 5 hairspray bomb with the second amendment attachment for ammo replenishment. Also equip overdrives to increase OD damage.

In each mini game pick the one with most chaos points as this increases your overall score at the end. I believe 100 chaos points doubles score (100% score increase).

Also go the the objectives on each stage as they double or quadruple your score on each mission.

With the upgraded hairspray bomb you should be able to secure many kills especially in the final defence mission which should lead to a 1st or 2nd placing in the team.

If you play through the whole match (5 mini games plus night defence) along with completed objectives you may get over 40,000,000 points which unlocks 5 spins at the end.

This is essentially 4 chances (apparently amps cannot be won on the 1st slot) to unlock an amp. If you unlock an amp upgrade the achievement will unlock at the score calculation screens and the upgrade itself appears as a black/white icon on the slot machine.

This isn't guaranteed but is an efficient way of getting the most spins per match. I managed to get an amp upgrade after 23 completed matches and unlocked on the 5th spin.