After 200 hours of gameplay, I think this is my favorite CT 15 solo build. This build can solo anything as the build relies on AOE freeze + Shatter. The more enemy to fight against, the better this build works. It can easily do the hardest CT Frontlines and Paxian Homestead in under 8 minutes.
Weapon: Assault Rifle Tactical Variant atleast 85k+++ I suggest re-rolling the Earthborn Renegade rifle until you get 95k+++ do check youtube on how to reroll that rifle.
Weapon Mods: Shatter, Icebreaker
Armor Mods: Spare Mag, Crit Stack, Toxic Lead, Mitigation From Death, Sharp Eye, King Slayer, Euthaniser, Blood Lust, Ice Trap, Personal Space.
If you are having problem with survivability, switch Ice Trap with Golem Stance. You don't need freeze damage boost as enemies die quickly because of shatter damage. You will have problem with Captains as they dont die fast hence, King Slayer or Captain Hunter is essential.