The train stage gives you lots of torque ammo, and even that, 10 is more than enough. I chose to use the assault rifle, but it really doesnt matter which way you choose.
As soon as you get in, stay in the light, and hunker down at the first barricade. Fire your torque, switch to your other weapon, and light him the hell up. When the kryll start to shield him again (or your AR clip runs out) reload your AR, then Reload your Torque, fire, rinse, and repeat.
If you've got a good spawn itll take him ages to walk toward you and you can nail his ass in several cycles.