There are only two possibilities:
1) The guard uses a special AoE ability called 'Riot Control' and everybody dies.
2) The group survives Riot Control, manages to bring the guard down to 0% - but the guard doesn't ever die. (Likely comes with player client crash.)
The solution is to down him the way EQ players attacked the Sleeper. Have a raid of a few hundred people constantly zerging the guard with damage with a few people off to the side spam resurrecting the fallen as even more people rush in to pick up where the others left off and keep the guard in a perpetual aggro state.
Since the guards are so powerful, why don't we just send One guard to Cyrodiil. Sure the guard would definitely win the war for our alliance. Instead of guarding a post. An empty post wouldn't be a big deal. Or we could organize a large v16 team to guard that post, and reward them with Alliance points.