Step 1. Don't enter the Mig-21
Step 2. Enter the Mirage 2000C
Step 3. Engage the F-5 and win.
XD
Naah, jokes aside my way of doing it, is going low and try to get to their back side so i cant be spotted easily or at all sometimes (the Mig-21 is really shiny) and use Heat seekers, 2x R60M's on each side of my wings and an external fuel pod on center pylon. The Mig-21 is all about speed and energy so, like Zomba said, use hit and run tactics.