To use the Nona, you have to click T to deploy. After deploying use the right click to send the mortar wherever your reticle is aiming. You can send that fucker half a map away honestly, the range is incredible. BDRM is worthless against anything GDI related.
To counter the Anti Tank Commanche, just fire an ADATS rocket at it while you're in a group rush. Designate yourself to being Anti Air watch while your team is moving out. Stay in the middle. You can also get a Hover MLRS and if you're lucky enough maneuver underneath the Comanche so it has to readjust and then kill it. Other than that, it's pretty difficult.