Try to place the specimen in a drop of hot Gelatin, on a clean glass slides under a suitable power of a binocular stereoscopic microscope and cutting transversely by a sharp razor with the help of a needle
We have recently published a boo chapter on guard cell isolation- very extensive and easy to follow.
Zhu, M., Jeon, B. W., Geng, S., Yu, Y., Balmant, K., Chen, S., & Assmann, S. M. (2015). Preparation of Epidermal Peels and Guard Cell Protoplasts for Cellular, Electrophysiological, and-Omics Assays of Guard Cell Function. Methods in molecular biology (Clifton, NJ), 1363, 89-121.
Thanks a lot for your answer. We have developped since that time a good technique with the imprits method, but I'll for sur read your paper. Thanks again Biswa.