How to escape the skulls (mist unit)

The Skulls can be avoided in their first appearance (Resucing Miller / Phantom Limbs) with a little bit of patience.

First, use your IntScan to mark all 4 Skulls -- two at the top of the bridge, and 2 along the slope to the left of it.

Now, you wait. The skulls will slowly shamble forwards, with the left skulls going down the slope, and the right skulls approaching and then beginning to go down the slope on the far side of the bridge. Your approach should be straight across. Once the right-pair has started down the slope, make your move. You have a rather narrow window where the Right skulls are far enough down the slope to break line of sight before they come back up the hill and camp the edge of the bridge.

Once you're a good 100m on the far side of the skulls, feel free to whistle up D-Horse, fling Miller over his back, and casually ride the rest of the way to the extraction site.

Edit:

Alternatively, change the helicopter's landing zone. If you take out the radar dish, you can have your chopper land right outside the prison. (Just make sure the guards are unavailable/incapacitated!)

First encounter (Extraction):

I marked the Skulls, headed down the hill towards the valley/bridge, went under the bridge (left to right), and then waited until the two Skulls on the bridge had crossed half way.

After that, I happily galloped off to the LZ.

Second Encounter (Honey):

After the cutscene, circled the compound counter-clockwise at the edge until I came to the area you can climb up. Climbed up, the end of the mist was extremely close. Didn't encounter any Skulls closer than 20m.

Skulls react to noise just like any other enemy. I went up top so that I was on the opposite end of the bridge from them and starting creeping forward. I then threw some empty magazines to their left (my right... since that was the side of the bridge they favored) so that they'd turn away. They seem to have terrible sight lines and I crouch-walked right past them. No waiting required.