I love the Holly Prime. For those unfamiliar with the load out it has:
1 Gauss Rifle
4 SPL
3 extra tons
The Gauss Rifle has exceptional range (hitting out to about 950m), while the SPL can rack up damage very quickly against anything that gets within 200m. If you manage to slam the gauss and all 4 SPL into a location, it is HURTING.
Yes, the Gauss Rifle is incredibly easy to hit, but the gauss also takes long enough to reload that you should be torso twisting to the right away from enemies while it reloads in order to protect it with... your head!

Seriously though, the jump jets let you dodge LB20X shots at close range so they catch a location other than the Right Torso or miss completely if you are lucky/good at timing it.
The Holly just needs time to land 2-3 shots into something and it is toast. Stay at range, radar passive and WATCH the battle (you can still see friendlies on passive radar so watch their armor, see who is engaging and then look that way. Find enemy laser fire, missiles, etc and follow them back to the source then zero in on the target and start laying the hurt on them. You are a direct fire support mech and when a gauss slug slams into someone from out of nowhere, people panic.