Infiltration: As was stated earlier, mages can't always be on the prowl, and even roving spirits must have some kind of search parameters to key on. That means legwork is your best friend there. Once you know what the mages and spirits look for, the best way in is to blend in, and not give them a reason to assense you.
Surveillance: Ultrawideband Radar or Ultrasound sensor. Explicitly works against Invisibility.
Spirit Defeat: Big guns, APDS ammo, going off weaknesses, overlapping grenades, MRSI, whatever it takes. In other words, put a big enough hole in it to overcome ITNW.
Killing the feckers: See above, except without the reference to ITNW. Remember, geek the mage first.