I think you'll find in practice that is far more difficult then the likes of YouTube make out.
Like trying to put the mine on the back of a Bastion.
I was trying to find a slightly safer way if not getting such high results.
Her armor is the problem.
You're very unlikely to get damage points shooting it without a Hullcracker or explosives to break the plates first.
Since you have a limited loadout, let your teammates, or if solo, other more suitably equipped randoms break the plates. If you have explosives tho, get in there and toss them. Make sure you have cover very nearby.
Don't waste ammo helping them strip the armor; just wait until you see a plate fall off.
Once a plate drops, dump everything into the exposed area. The red core near the face is the best target, but even unarmored legs will award points.
Alternatively, if you can't wait for a plate to fall you can try for the yellow weak spots under her legs (similar to a Bastion), but those are
very difficult to hit, and getting the angle on them is going to probably put you closer than you want to be.
If you're running anything other than hull cracker or simialr you need other players to expose a weak spot for you. If there arent any then it's better to move on.
Once the plates are down you can do dmg numbers even with light ammo weapons. Just make sure you target the exposed areas only.