I don't understand why, if these are the 12 people he wants dead, didn't Donflamingo just use Parasite string on them.
He clearly tried to use it on the Admiral, so it's not like he'd think they'd be too strong for it to work on them.
It's not about finding someone easier. It's about finding someone that's more complimentary to his skill set.
It's like back in Enies Lobby. Sanji didn't need to find someone easier than Califa. He needed to find someone he was able to fight.
Likewise, Zoro needs to find someone that's not immune to being cut.
But he already CAN cut Pica. It just doesn't do anything.