。
I'll probably have finished the work by tomorrow.
Literal
Tomorrow [by-までには] work [object-を] finish-completed-will-be [probably-でしょう].
やり終えている combines やる (do) + 終える (finish) + ている (resultative state) — 'will be in the state of having finished.' ~でしょう adds probability. The compound verb layering (do→finish→be-in-state-of) is characteristic of how Japanese expresses future perfect.