Welcome to this quick guide to get the 4 co-op exclusive achievements for Lara Croft and the Guardian of Light.
Using this guide you can easily get all 4 achievements in ~5 minutes.
Required are:

• 2 input methods (e.g. a controller and a keyboard, I personally used two controllers)
  
• Access to the second level in the game, Temple Grounds (unlocks from single player carry over)

This is the easiest one to get, just fire up a local multiplayer game, if you're going for the other achievements as well I suggest level 2 (Temple Grounds).
The achievement will pop up as soon as the level has finished loading.

Another easy one, Totec has two ways to hold his shield. In front of him (same button as the Grappling Hook for Lara, so R1/RB) and above him (L1/LB). Hold the shield over Totec's head, jump on top of him using Lara and then jump with both characters at (roughly) the same time.

This takes some coordinating and a cliff. In level 2 make your way down the plateau you start on and to the right to find a lethal cliff face. Position Lara on the edge and get ready to hit her Grappling Hook button (R1/RB on controllers). Then just jump off with Totec and use Lara to save him.
Don't worry too much about getting this right in one try, since there's a very generous auto-respawn.

Apparently just jumping near the cliff and grappling Totec should also trigger the achievement.

Another easy one. From the cliff face in level 2 you used for the previous achievement move down and left, following a path. A cutscene will trigger after a while (there's two easy fights before then. Remember that it's fine if one of the two characters dies) showing ancient monsters with ranged attacks.
Perfect, just what we needed. Hold the Grappling Hook button while controlling Totec to raise his shield in front of him, then just approach the enemies. There's no specific need to aim the reflected shots since they seek out a target on their own.