Rare Mexican Tremroids I say.
However, there are some givens with mara. If a mara that NEVER missed somehow came across a chainsuit with a lock-on that did not stray from the target (*cough* LeStrange), the mara WILL lose. It is just flat out a given. I have not fought a chainsuit that can aim well and won without having the advantage of sneaking up first, because they just do too much damage too fast. With basi though, I've killed many chainsuits because basi can be a hell of a lot more elusive to those who lock on, plus it can grab, and circle and feignt while getting headswipes.
But yeah, there's a point where maras just can't really be good enough to take down some s2 humans. Apple owns me terribly with rifle, I killed him maybe 2 out of every 10 fights we had last night in a devmap training session. He just aims too damn well with rifle against mara, and it dooms them. That, and he shows that outside at ATCS is even deadlier for marauders, because a human who can circle jump well can get distance enough from a mara that a jump to catch up with the human is almost always fatal for that mara.