I have been in this position several times, made it to the end, reasonably powerful position and no idea whatsoever of how to close the game down. My end game sucks in MP.
Possibly your most potent weapon is diplomacy, try and get everyone to gang up on Marignon in 1st place, then if you finish him off, gang up on the 2nd place empire etc...
Your golem SC is probably your 2nd best weapon, get astral tempest, cast ritual of returning on them, try to give them defensive items such as awe, fear, protection, defense, then teleport them in and hurt large armies.