NO player character is invincible to a proper GM.
Remember the old adage of "if you stat it they will kill it"?
Well guess what player characters are made of?
They have stats; you can kill them.
Or failing that, collapse the multiiverse on them.
This approach has many traps that are going to eradicate the fun from the game: from the usual gearing-the-NPCs-against-the-PC adventure (also called PC/GM War), to the dying screams of Fluff Fred: "I just wanted to have fun!" as he gets his butt blasted into another universe by a multi-melta because that's the only weapon that can harm MinMax Mike, so every combat encounter has at least one.