The speaker uses that old-fashioned "O" again. (Kind of starting to sound like Shakespeare, right?)
The speaker's "kinsmen" are his buddies; they are like family to the speaker. This may be because they are fighting against the same "foe," or enemy.
Though far outnumbered let us show us brave, And for their thousand blows deal one deathblow!
The speaker is really starting to get his allies pumped up.
Their side is the underdog, and has fewer people than the enemy does, but that's not going to stop the speaker from acting like a hero.
All he wants is to get one good shot at his enemy.
This shot, or "deathblow," is like that one punch that a losing boxer gets in at the very end of a movie. Sometimes that one punch can win the boxing bout, but it doesn't work like that in real life. The speaker still expects to lose.
What though before us lies the open grave?
This line could have been taken straight out of one of Shakespeare's famous tragedies, like King Lear, Macbeth, or Hamlet.
If we put the line in modern language, it comes out to be something like this: "Is there anything else in store for us but death?" Morbid, we know, but this is some serious stuff.
The speaker has accepted his fate; it's not like he planned on living forever.
Like men we'll face the murderous, cowardly pack, Pressed to the wall, dying, but fighting back!
We get to the last sentence, and it's time to wrap things up.
The speaker urges his allies to be manly.
Putting the outdated gender roles aside, manliness is associated with being brave, noble, and tough. Being "men" also contrasts with the first line where the speaker talks about not dying like hogs.
The speaker says they'll stand up to the dog-like enemy, even though they are trapped against the metaphorical wall.
And even though they'll surely die, they are going to fight back.