Pale beds of blowing rushes
Where no leaf blooms or blushes
Save this whereout she crushes
For dead men deadly wine.
Pale, without name or number,
In fruitless fields of corn,
They bow themselves and slumber
All night till light is born;
And like a soul belated,
In hell and heaven unmated,
By cloud and mist abated
Comes out of darkness morn.
Though one were strong as seven,
He too with death shall dwell,
Nor wake with wings in heaven,
Nor weep for pains in hell;
‘Even if someone had the strength of seven men…’ 
 The ‘were’ is a use of the  subjunctive mode or “mood” , which is becoming increasingly rare in colloquial English. 
 ‘seven’ is probably used for the sake of rhyme, but it also has great significance within (and without) the Bible, as Swinburne and his contemporaries would have known. The poet himself had long ago lost the faith that he’d learned as a child & so the line may have a bitter underwriting: ‘Even though a man have great faith in the Seven (days of creation in  Genesis ;  declarations of divinity  in John; seven trumpets to herald the destruction of Jericho in  Exodus , etc.), it will not spare him from death & eternal night.’
The Garden Of Proserpine