1 This was that woman, this that deadly wound, 2 That Proteus prophesied should him dismay,
dismay > defeat, overcome
3 Which his mother vainly did expound
vainly > in vain; also: foolishly, hence: erroneously
4 To be heart-wounding love, which should assay
assay > attack; assail [him]; or, more likely, in view of the absence of a comma at the end of the line: essay, attempt
5 To bring her son to his last decay.
decay > downfall, destruction ("last decay" = "death")
6 So tickle be the terms of mortal state,
So > Thus tickle > capricious, fickle; precarious, insecure