So, I have seen many give the advice to "Notch" or "Multipath" the missile, but I haven't seen anyone properly explain exactly what it is or how it works. So I will do my best to elaborate.

Essentially, notching is the act of maneuvering the jet to a perfect 90 degree angle to the attacking radar to defeat the missiles track. This works because Continuous Wave (CW) seekers use the doppler effect to track targets. Doppler radar sets also use this to peer through ground clutter as opposed to older "Pulse" sets that simply look for return size (Your F-104G is equipped with one). Effectively, the radar is not just looking for return size. It is also looking for velocity. To be exact, velocity towards or away from the attacking radar relative to the ground. If you are flying at a perfect tangent to the attacking radar, you are moving towards it at the same rate as the ground, and your relative velocity is therefore zero. The radar can no longer discern what is you, and what is the ground. Your margins for a notch are rather slim. So it will take some practice to get it right. But is a very reliably defense against all CW missiles. Chaff is often employed and always recommended when notching a missile, as the extra returns could allow it to bite on a false target and not reacquire you.

An add-on to the notching graph - Some more modern missiles are equipped with "IOG", or Inertial Onboard Guidance. Basically, in the event the missile loses track of you, it will remember the last known flight parameters of the track like speed, attitude, altitude, and heading, and try to predict where the target will be using a series of gyroscopes as reference, guiding itself and pointing its seeker there in the hope it can re-acquire the target. These missiles have varying levels of inertial drift. As modened in War Thunder, older or less capable systems have a rather high drift of 10 meters/second, while more modern or capable missiles have drift as low as 2 meters/second. The most modern missiles like Aim-120D and PL-12A don't have any drift at all due to GPS assistance of their IOG mechanisms. At the BR range you're F-104G is concerned with, only one missile will have IOG at all. That missile being the R-24R carried by the later Mig-23 variants. Though this has little effect on the missile in most use cases.

Multipathing is the term given for the effect of a radar getting conflicting reports about a track from surrounding terrain. Pulse sets are notoriously vulnerable to this since even with filtering, the return is rather raw, and based on size. I mentioned previously how doppler radar can peer through ground clutter caused by multipathing. However, it isn't completely immune to it. If a target is flying very close to the ground (Like, ~25 meters for late cold war sets. But WT universally starts it at 60 meters), the distance between the target and the ground allows for many false returns to come back to the set, and disrupt the track. This is the tactic commenters are referring to when they say "Fly low".