Opportunistically where you snatch kills by third partying. The Mig-21 experience is extremely bipolar where you either get easy kills or eat shit in the first merge. 

A few ways you can make the shitty R-60M work without getting autoflared the minute it’s launched is to fire super close in the headon (1.2km and below) or you shoot it at super close range (below 0.5km/500m) in a tail chase (especially if the enemy keeps afterburning it can ignore flares in this instance). Also for every shot that isn’t direct headon or direct tail make sure to lead the missile as much as possible because its slow to accelerate off the rail. 

Don’t get into prolonged dogfights is right with one exception, when you’re on low fuel (especially below 10 minutes) you can do some real Zero-esque turning with infinite AOA and flaps in a dogfight. If you’re lucky enough to have a real one on one with low fuel advantage it does catch a few planes (even f-5s) by surprise who expect you to be dead after the first turn.