best option as has already been stated: Dont be solo. Be in a group with others that can potentially hit it, someone should be at some angle to hit it at all times.
2nd best option: Keep an eye on them... they generally take some time to line up, so you'll usually know where it's going to be coming from. Maneuver yourself to get an angle on it. It's pretty predictable that a mech would run for cover when attacked by a sulla, so they'll already know how to line up on you next if you're hiding behind a mountain. A good sulla pilot will simply use that against you.
