how are the death Busters suppose to know that Ranma's focus is wavering? They don't live in Nerima and they didn't get Happosai's leaflet.
Almost every person I can recall the DBs targeting usually spends a minute or two being studied by Eudial or whoever before the attack takes place. If Ranma isn't showing the focus on the martial arts that he's famous for, Eudial might assume she's got the wrong person and call off the attack. Alternately, if Ranma saves Hotaru (or vice versa) the ensuing interaction can help Ranma obtain the resolve he needs to face Happosai again. He then loudly proclaims his intent to be the best again, then Eudial jumps out of the bushes where she's been evesdropping and commences the attack.
As for how they know of Ranma in the first place, he's more or less a household name in Nerima; surely some of that fame would trickle over into the other wards of Tokyo, the way the Senshi have become urban legends all over the city.
*EDIT*
Found the moxibustion arc. The manga seems to contradict itself as to how weak Ranma is during this period. He's as fast and tough as ever but a punch meant to vent his wrath on a wall didn't even phase the toddler he accidently hit. In other words, in a narrow alley with little room to maneuver and a bunch of thugs, probably bearing knives and so forth, a weakened Ranma is going to have his work cut out for him. I'll keep examining the Ranma = victim of the week angle, but I think my original idea still holds water.