6-6-6-1
0-0-6-6
3-0-6

This is my standard pve build. From this build, if you're looking to optimize damage, you would obviously switch out the 6 in protection for 6 in sonic boom. I choose not to because I like the 60 armor to party.

To any engi putting 6 points in pain, you're basically wasting them. The only thing that increases with rank for that skill is initial hit damage and it's a tiny negligible increase.