One of the final optional missions you will receive upon reaching the endgame. Take your time on this mission and keep a close eye on the health of your troops. Save artillery strike and orbital bombardments when the Eldar reinforcements arrive. There is a relay very close to this fight, so withdraw and reinforce your compliment as needed.
A very easy solution to this fight is to kite the boss using Avitus, as the Eldar will not leave the immediate area.