There has been a lot of rocket mortar users recently roaming the game. A lot are complaining about rocket mortar users. There are two valid ways to absolutely kill rocket mortars:
1. One arm rocket mortar 12 + missile rack 12 enchanted by rocket mortar 12 implants which I am using.
2. Aegis dome shield + any guided missile (I have used Javelins) which I have tested out and worked flawlessly.
The third way I suggest is using killshot/surge/shadow with sniper weapon. I have never tested it out but here is a sample:
