This again from long ago, i think I read it in the seventies but maybe later. It was one of the first times I came across the concept of motherships and star carriers, before that space battles in books always seemed to be between massive Lensman type ships.
I remember some admiral type at his plotting screen giving out orders to individual commanders (like Ender Wiggins) but then he had to wait due to relativity to find out who won.
He exclaimed "Brilliant" when he watched the video of one of his fleets and , just as it neared the enemy, this guy ordered out all the small ships boats, i think each craft carried a dozen or so, and these little space boats had one charge each of "starfire" which was some kind of plasma beam.
His fleets won but as he was returning to base they detected incoming at multiple light speeds. This proved to be two intergalactic elfs (not elves but elfs) who proceeded to use him and his men as game players.
They found themselves transported to a dull grassy planet and dressed in chainmail with swords etc. The much larger alien contingent was approaching in the distance so a forced march to a defensible forest area. Thanks to some clever tactics they were winning so the other elf started cheating. Suddenly all the trees vanished so his men were out in the open and were getting slaughtered.
Then the elf who had the humans in sway gave all his men 'blasters' so the other elf gave the aliens force screens and their own blasters. The general got hit and the action froze.
The two bickering elfs strode about the battleground and then reincarnated all. I think then some high energy alien arrives and gives our hero a bit of psi power so he can confront the elfs. They beg for mercy but he chants the word/mantra "Om" two or three times and that traps them forever in an oubliette dimension. He then goes back as the hero to the empire. His name started with 'R' (but not Ransom or Retief as have been suggested)
This was a 'medium size' novel, I'm fairly sure it was "hardback"
Note: I asked this a while ago in Chronicles but nobody could help https://www.sffchronicles.com/threads/567453/