I haven't played these guys in a long run, mostly because of the Lizbeth ban. I may try them again this week because Mona is unbanned and I need something to counter those Montana.
Which are the clans they have the best synergy with? Should I go for a defensive, offensive or pill manip half? Selma, Ahkab or Bloodh?
Piranas have excellent openers but almost no characters with abilities that are consistently good in the last round; that's one of the clan's weaknesses, and coupled with low damage means most likely you wont win the game by KO. Especially in 5*, that's why Lizbeth was so great since she actually can be used last; without her the only Piranas 5* that makes most sense to use last is Bloodh.
Tyd serves two valuable functions in Piranas mono; first you can use him last for his 8 power, and second he fights SOA which ruins nearly all other Piranas characters. Tyd is only good in mono though; since he has no ability, you don't want to ever draw him without the clan bonus.
Raskal is great for Piranas halves; like I said, he has basically the same build as Diana. However, in mono it is not worth it to open with Raskal or play him early instead of a pill manipulator like Dalhia Cr or an attack manipulator like Taljion. So he seems to fit nicely instead in Piranas halves where you have less consistent pill manipulation options, and where you can find closers in your other half.
Piranas mono have a wealth of options to open, but needs su; Tyd is one of the best closers in clan; mono doesn't need another 4* opener like Raskal. Piranas half is very different- Raskal's raw stats and above curve ability make him well suited for halves.