Why did they change the method?

So they destroy all other starfighters by sending some bombs/meteors/torpedoes/missiles/whatever, and it works.

Then they change the method by sending inept, incompetent, bumbling fools (can these 'ridiculously deliberately ugly' monsters even be called 'assassins'?) to basically not-kill Alex.

Why? Why did they change the method from super effective and 'impossible to defend against' into 'very easy to combat and escape'-type?

Why not just send a couple more of those missiles to the trailer park and wipe out the whole thing, with Alex, Beta Unit and Centaur. Problem solved.