SEGA Mega Drive Ultimate Collection

Work your way through the Sega MEGA DRIVE titles, you don't need to play them for long, you can boot them up and quit out at the menu, but you do have to "play" them all in one sitting.

*degen8 has informed me you don't need to do this all in a single sitting.

After playing through all of the games to unlock their respective achievement, you will only need to load the following games for this achievement;

Dynamite Headdy
Gain Ground
Phantasy Star 2, 3 and 4
Shining Force and Shining Force 2
Shining In The Darkness
Sonic & Knuckles
Sonic The Hedgehog 2 (Though, STH2 is required for "Only In The '80s". If you've unlocked that achievement, you don't need to play this again.)
Streets of Rage 2

Here's what worked for me;

I loaded up each one of the Sega Mega Drive titles. I didn't actually play them, simply getting to the start screen was enough. I found doing them in alphabetical order was the easiest way to keep track. Simply load each title until you get to the start screen for that game, then exit the title using the back button... then sellect the next title... keep going until the achievement unlocks.

I had already played some of the titles when I went for this achievement, so to start with I only loaded/played the one's I hadn't already previously played. The achievement didn't actually unlock until I had loaded/played every title in the same session. Even by playing/loading each title, this achievement shouldn't take any longer than 10-15 mins.

Hope this helps.