Generally, it's easier to deal with things after you've been on the opposite end of the barrel a couple of times and know what's hard to deal with as a tanker (for example) first-hand.

It may seem futile to try shooting back at the tank as a heavy at first, but after you've been in the tank's situation you know more about how to position yourself as the heavy to make the tank's life harder. You know the importance of constantly changing position of where to peek out and shoot at the tank. And you know where a sundy position is slightly better or worse, as it is harder or easier for the tank to get at, with for example safe spots for heavies in mind. All these things comes intuitively without even thinking about after having played for a while.

Also, in some situations there's simply not much you as a single infantry player can do when an MBT comes rolling to take out your sunderer so don't feel too bad. Maybe if you're lucky, but otherwise it's just hoping your side has more spawns and that more people will be reacting to help defend the other spawns. In some situations you can scare the tank off for long enough that you get some help and repair the sundy. Most people are afraid to lose their MBT and after a couple of rockets they'll rather retreat than stick around to destroy the sundy while also risking their tank.

If the sundy dies, you need to make a quick decision wether to run to the point and try to last out the timer, or redeploy back to last base where you either pull a tank yourself or a new sunderer.

But bottom line is that you will learn these things as you play. There are so many factors involved that it would be impossible to mention them all. Even just learning to identify the engine sound of an enemy MBT will reduce your reaction time and enable you to switch to an appropriate class faster and perhaps get an extra rocket or two on the tank which makes alot of difference.

Anyway, good luck!