His owne perill on his forwardnesse: i.e. let his danger be blamed on his own presumption.

Cousin: Duke Frederick addresses Rosalind with casual familiarity, but in the next line Rosalind responds with the utmost formality: "Liege".

there is such oddes in the man: Orlando is obviously no match for Charles; the odds are strongly in Charles' favor

faine: like to

generall challenger: challenger who will take on all opponents 

misprised: dishonored

suit: plea

supplied: occupied

eeke: add to, increase, supplement

I be deciev'd in you: underestimate your strength

one fall: one round

entreat him to a second: Duke Frederick could not persuade Orlando from the first fight, but Charles' might will injury Orlando so greatly that there will be no chance to entreat (persuade, convince) him to a second round.

Hercules, be thy speede: Hercules is invoked as a demi-god of incredible strength and speed. 

thunderbolt in mine eye: It was popular in romantic love poetry to claim Jove's power for the eyes of a mistress; Celia is calling to the gods for divine intervention to help Orlando. This is in contrast to the previous line from Rosalind, "Oh excellent yong man" which indicates that Orlando is doing well, between that line and Celia's Orlando seems to have lost the upper hand. 

well breath'd: adequately exercised, fully extended; Orlando is saying that he could go another round, he has yet to reach his full potential.

I would thou hadst beene son to some man else: Duke Frederick changes the tone and rhythm of the scene by changing to speak in verse here.