FINALLY FIGURED THE QUICKEST SOLUTION FOR THIS.
1. On your PC, make a folder that contains the video/s you want to transfer into your Camera Roll.
2. Mix some pictures in it, 2-3 will do. (Note: Don't put in too much pictures because you'll have to sync the whole folder later on.)
3. Sync your iPhone to iTunes.
4. On the "User's iPhone," select the Photos tab under Settings.
5. Check Sync Photos.
6. Click Copy photos from, then locate and select the folder where you saved the video. (Note: The one you made from Step 1.)
7. Back to iTunes, you'll see the second box that has All folders, Selected folders, and Include videos. Check the Include videos selection.
8. Finally, SYNC and wait.
9. Unplug your iPhone and check your Camera Roll.
10. Find to see the video you transferred. (I found mine in the Videos folder.)
11. See magic. 😉
IF YOU HAVE QUESTIONS, PLEASE REPLY TO THIS MESSAGE. I'D BE REALLY GLAD TO HELP YOU. 🙂