A group of minions attack strength depends on the number of minions in the group. Does this mean their attack strength gets weaker as they start dying off? or does it stay the same till the last one?

It reduces the roll.

This is one reason why taking out minions is a good thing for 'less combat capable' characters to do while the bit warriors deal with the harder to hit nemesis/rivals.

Yes. Minion attrition means less skill ranks.

So say you have a group of 5 minions with Group Skills of Ranged (Heavy) and Perception, so the group has 4 ranks in each of those skills. They have Agility 3 and Cunning 2, so their skill pools for Ranged (Heavy) start out at PPPA, while Perception is at PPAA.

Kill off one minion, and you reduce the skill rank to 3, so the pools drop to PPP and PPA, respectively.

Kill off another so that you're down to 3 minions & 2 skill ranks, and you're rolling PPA for Ranged (Heavy) and PP for Perception.

Remove another minion, it's PAA for Ranged and PA for Perception.

And if you're dealing with the last lone minion, he's now just rolling his characteristics using the (green) Ability dice at AAA and AA.

No skills for you!!

Edit: yellow is hard to read!

This is also a reason for having large minion groups. A squad of eight stormtroopers doesn't start to lose any effectiveness until it's reduced below six stormtroopers.

Minions also only do this skill rank thing in skills they have noted against them under the Skills (Group Only) section. if they have no skill listed then they upgrade no checks no matter how many are in the minion group.

This can be helpful if you would like to give the pc's a sense of being over run.

a good example of this would be something like a pack of hyenas, or the local townsfolk lynch mob.

giving this non combat group a skills that are non-combat can be fun too, such as Cool for a predator, or discipline for the crowd of people (a rioting mob is hard to breakup, but separate them and they soon give up)