It depends on how you want to rule the summoning circle. A circle is a two-dimensional shape and the marilith is a three-dimensional creature. Is there any reason that she couldn't go up or down?
Since she's trapped inside the circle, that implies that she herself was summoned. If that's not true, perhaps she should summon some help?
Otherwise, I agree with the blade barrier and telekinesis approach.
Yes, the marilith was summoned...