Stranglehold

Use the Barrage special move with every weapon.

Kill 100 opponents as John Woo in Multiplayer matches.

I was playing this game on Hard and I got all the way to the second half of the final boss fight before I finally gave up and lowered the difficulty to Normal.

I got the achievement for "Normal Completion" which surprised me a little because I only beat the last part of the game on Normal.

The next time I clicked "continue" with the intent to beat the final battle on "Hard", and it just went to the final cut-scene and I got the achievement for "Hard Completion"!

I tried it on "Casual" and got another achievement. It seems the only difficulty this wouldn't work on was "Hard Boiled", but I'm certainly not complaining. I'm not sure if this works if you beat the majority of the game on lower difficulty levels, but I just wanted to share this with the world.

If you load that last checkpoint via Continue game, you can get the 'Old School' achievement as well.

Kill 50 opponents while in Tequila Time in Multiplayer matches.

Complete the Chicago History Museum level on any Difficulty setting.

Kill 10 enemies with a 5 star kill rating.

When using the Barrage special ability in single player, your character is both invulnerable and has infinite ammo; if you need to cut the ability short to conserve the Tequila Bomb meter, press the D-pad RIGHT again to turn off Barrage mode. The unused Tequila Bomb energy may be conserved for later use.

Kill 50 enemies by using the environment instead of guns.

Kill 50 enemies while hanging from a chandelier.

Although you can simply run across the crumbling areas of Chapter 6 Slums of Kowloon for the achievement, you can also load the checkpoint "Condemned" in chapter select and shoot-dodge to the corner where the Heavy Machine Gun is and grab the achievement.