Get on top of Mercs boathouse roof
1. Go up the stairs and jump on the ledge that sits to the right of the door
2. Slowly walk to your left untill you get to the lower ledge
3. Push up and left on the left analog stick and jump

Skywalk at Sawmill
1. Get a Rpg/at4 and set it to quick select
2. Go up the ramp that leads to the water and goto the edge
3. Open inventory and highlight the rocket launcher
4. Walk forward and hit x, you should walk in the water while your rocket is coming out
Walk back up the ramp, you should now be in the rocket stance, walk off and you should be doing the skywalker