This is the lesser known stanza that is not publicly translated. It comes from a (in my opinion) more important list of graves which include Gwydion, Mabon and Merlin.
Bedd Llew Llaw Gyffes
Dan achles mor cyn divot y armes
Gur oed ef gualioc mei ormes
The Grave of Llew Llaw Gyffes
Under protection of sea beyond destruction of the prophecy
He was a man embattled (walled) in the middle of oppression (attack, violence, destruction, plague, etc)
This isn't entirely dissimilar to a comment made about him elsewhere which says he was a man who invited attack. However, it may also link him to immortality. Consider that the destruction of the prophecy is likely reference to a cataclysmic event, whether it is looked at through a pagan or Christian lens. It also may connect to the other more widely known grave stanza regarding him which reads as follows: Bet Llev Llaugyfes y dan achles mor, yn y bu y gywnes, gur guir y neb ny rodes.
The grave of Llew Llawgyffes under the protection of the sea, With which he was familiar He was a man that never gave the truth to anyone
Under the protection of the sea would seem to be a similar idea as the Isle of Avalon or Tir na nOg being beyond the sea and a place of immortality. However, the difference here is while the first mentions prophecy, the second mentions that Llew was deceptive or dishonest. The two ideas may be connected in relation to the god's connection with prophetic declarations, as evidenced with Lugh in Irish tradition.