No More Heroes: Heroes' Paradise

You can start assassination missions at K-entertainment, which is marked as a skull on the map. In order to unlock all assassination missions you will have to attain a silver or gold medal on side-jobs. Refer to You’ve made it into the Hall of Fame of my heart! for more information on the side-jobs.

There are 25 assassination missions:
Assassination Gig 01 – Pizza Butt ( Gold medal: Finished in 15 seconds or less)
Assassination Gig 02 – Snake Hall ( Gold medal: Finished in 1 minute or less)
Assassination Gig 03 – Hooligans Parade (Gold medal: Killed all 15 enemies)
Assassination Gig 04 – Baseball battle (Gold medal: Killed all 9 enemies)
Assassination Gig 05 – Guard break (Gold medal: 20 kills)
Assassination Gig 06 – Pizza Butt Returns (Gold medal: Finished in 15 seconds or less)
Assassination Gig 07 – Sword Master (Gold medal: Finished in 1 minute or less)
Assassination Gig 08 – Power-Down Battle (Gold medal: Finished in 1 minute or less)
Assassination Gig 09 – Death Match 100 (Gold medal: Killed all 100 enemies)
Assassination Gig 10 – Baseball Battle round 2 ( Gold medal: Killed all 9 enemies)
Assassination Gig 11 – Dark side battle ( Gold medal: Killed 20 enemies)
Assassination Gig 12 – Tiger Hall (Gold medal: Defeat all enemies)
Assassination Gig 13 – Pizza Butt: The Finale (Gold medal: Finished in 15 seconds or less)
Assassination Gig 14 – Big Bang Anarchy (Gold medal: 100 kills)
Assassination Gig 15 – Pro-wrestling Contest (Gold medal: Defeat all enemies)
Assassination Gig 16 – Underground (Gold medal: Finished in 1 minute and 30 seconds or less)
Assassination Gig 17 – Baseball Battle Round 3 (Gold medal: Killed all 9 enemies)
Assassination Gig 18 – Extreme fight (Gold medal: Finished in 1 minute and 30 seconds or less)
Assassination Gig 19 – Armed Hoods (Gold medal: Finished in 2 minutes or less)
Assassination Gig 20 – Shortcake Freaks (Gold medal: Finished in 56 seconds or less)
Assassination Gig 21 – Speed Jack (Gold medal: Finished in 1 minute and 30 seconds or less)
Assassination Gig 22 – Gamble Fight (Gold medal: Killed 65 enemies)
Assassination Gig 23 – Single Strike (Gold medal: Killed 7 enemies)
Assassination Gig 24 – Batting Killing Centre (Gold medal: Finished in 1 minute and 30 seconds or less)
Assassination Gig 25 – Crowded Train Carnage (Gold medal: Finished in 30 seconds or less)

Credits go to BreadSkin for supplying the requirements for the 20 original Assassination Gigs. I have verified these requirements in the EU version of the game and they are spot on.

The missions are pretty straight-forward and do not need further explanation.
Assassination gig 03 and 15 however, require you to kill your enemies by only using wrestling moves. The best way to do this is to hold the or / button for a charged melee attack which is 100% sure to stun your enemies.