The Shredders are a pain to get hold of. And Splinter isn't much better (I hate his buff move so much in the dojo event
)
Grinding through it slowly though, its just taking a Loooong time. In theory it is possible to get Shredder DNA from the Portal packs (the biggest one), but it takes ages to get the greenbacks to pay for them. Still if you can, then thats 25-80 Shredder DNA on its own!
Has anyone had any better luck than me with getting mutanimal or Classic/Mirage DNA? I've only managed to scrape together enough to get very low level Leo/Don (classic), Mikey/Don (Mirage) and pidgeon pete. I just find that I get slaughtered each time I attempt the Mutanimal event.