Eliminating the separate battery may help a bit, bit 5 minutes is not at all atypical with Zenmuse and a VTX. Many people feel that putting the VTX on a separate pack actually adds a bit of flight time. Remember, you are going to lose the weight, but then the VTX is going to pull current from your main pack, which on balance might make losing the weight moot. That said, I power everything from the main pack myself.
If you want to do the wiring it's fairly simple. Open the Phantom shell (Google for the procedure) and solder the power wires from the VTX (that are currently connected to your 2nd battery) to the main board right where the battery leads connect. (If you have a Phantom with the original main board (not the upgrade board) then there is a pair of AUX power leads tucked away in one of the landing strut holes between the mounting screws. You can also use those wires to connect to your VTX power.)
Since you mention being technically challenged, you may want to enlist a friend who is experienced with soldering. If you add leads to the main battery pads, you want to be absolutely sure the soldering job is top notch -- otherwise you may have a battery lead come off in flight and you can imagine what would happen next.
