This is a Metal Gear Solid 2 achievement.
MGS2 achievement. To get this, you need to do 200 pull-ups while in hanging mode. 100 pull-ups will get you to level 2 and 100 more to level 3. To execute a pull-up, enter First Person View mode and press/hold the item and weapon selection buttons while hanging. Watch for your grip gauge so you don't fall. The pull-ups don't need to be consecutive; you can climb back up and hang again if your grip gauge gets dangerously low.
To make sure you're doing the pull-ups right, you have to hold the buttons until your character does a complete motion. Just tapping or letting go of the buttons too early will trigger an incomplete motion and doesn't count as a pull-up. When you level up, your grip gauge will indicate it by changing colors and the title (Grip Lv. 2 instead of 1 etc)
Another way to quickly grip level up is by dropping from a higher ledge and quickly hanging in a lower one. A good place to do this is when you first enter the Holds (the first area where you can hear Scott Dolph's speech). Instead of taking the ladder down, hang from the ledge, drop and press the hang button again during the fall immediately when you reach the lower ledge. Snake should grab it and lose a great amount of grip. It will count as doing 10 pull-ups. Just be careful not to fall or you'll die.