*abrupt second 180*
BA NINJA TRICK
So, to clear this up so you understand what happened a bit better, BA don't have passive radar, they're just very hard to detect on standard radar because of the extremely small radar cross-section. Good BA players can use this to their advantage by firing weapons at you from beyond the range they can be detected (like the BLUE BA CANNON, otherwise known as the Manpack PPC), making it seem as though you're getting attacked from nowhere.
Tactics you can use:
1) Call for help. Certain Mechs/Vehicles are extremely effective against BA, and hopefully your teammates will be willing to kill it for you.
2) Watch for where the fire is coming from. If you were in a freshly repaired Mech, a single BA with a PPC isn't really a threat. Calm down, relax, and find where it's shooting from; use the appropriate weapons to take it out: Lasers, MG's, Splash damage weapons, etc. Also, look for Jump Jet trails if the BA is jumping, the trails tend to be very easy to spot.
3) If it starts to rush you, backpedal away from it. Most Mechs can outrun a BA even while moving backward, and BA tend to be much easier to deal with at range. By backpedaling, you keep it at range while limiting its lateral movement, making it much easier to hit.
4) Ditch the Mech and fight it out as a BA. This isn't really recommended if you're not good at BA fights/not familiar with BA's.
Just practice against BA, they've already been nerfed pretty hard, and some players simply view them as easy kills.