After I deployed my Hover Bikes (~20) and my flying cars many of the UFOs retreated
and only 2 brown UFOs and 3 pink UFOs remained. I downed the 2 brown UFOs and 2 pink UFOs, the other retreating. Now I'm attacking a downed brown UFO. Wish me luck.
BTW, they were trying to destroy my base. Every UFO was heading towards it, but their luck ran out.
@Sharp: I'm not bad at killing them, but I haven't been attacked 1 and a half week(s ?). (and I counted them again; there were only 9 UFOs; I looked on the minimap and saw ~15 because there were some Cult of Sirius vehicles around the city; I gotta attack them more often)