It is true that mara is much easier to out maneuver than other aliens, with the probably exception of the dretch, but it is hard to say that whoever is more accurate will win the fight, since there are variables in the damage each will do with said accuracy. Hypothetically, if you take a mara that doesn't miss headshots and a helmeted human that doesn't miss any shots, the mara will always win if it gets close, 100% of the time, unless the human is using a flamer, luci, or (depending on the distance) a painsaw.
That may be taken as too much of a stretch for an example, but my point is that the damages are different. If a human DOES get that close with two equally skilled opponents, the mara will, theoretically, have the advantage, accuracy aside.