Shall be constrained to honor us though dead! (8)
This could be lifted straight out of the Iliad or another Greek heroic epic, because the myth was that your afterlife lasted as long as living people remembered you.
And for their thousand blows deal one deathblow! (11)
This is a very macho attitude towards death. The speaker does not intend to go down a victim.
What though before us lies the open grave? (12)
This is the scariest line in the poem; Shakespeare would have used something similar in the climax of one of his tragedies. The sentiment here is that, since they're absolutely going to die, they might as well go down fighting nobly.