  We study the torus equivariant Schubert classes of the Grassmannian of
non-maximal isotropic subspaces in a symplectic vector space. We prove a
formula that expresses each of those classes as a sum of multi Schur-Pfaffians,
whose entries are equivariantly modified special Schubert classes. Our result
gives a proof to Wilson's conjectural formula, which generalizes the Giambelli
formula for the ordinary cohomology proved by Buch-Kresch-Tamvakis, given in
terms of Young's raising operators. Furthermore we show that the formula
extends to a certain family of Schubert classes of the symplectic partial
isotropic flag varieties.
