Even after an entire trilogy, it's hard to think of a more iconic antagonist in Mass Effect Legendary Edition than Saren Arterius. In the following games, the major antagonists are opposed indirectly, much like how Sovereign is engaged. Players don't get to fight their nemesis most of the time, but Saren is a welcome exception to this rule. Unless, of course, they can't beat him.
And that is sadly a common thread, especially for gamers who play the game on the Insanity difficulty. No enemy in the game is quite like him, which is expected, but because he lacks the weaknesses that other enemies have, players that breezed through the game can easily hit a brick wall with Saren. Don't quit on the final boss, it's time to finish off the game with some serious strategy that makes the fight a cakewalk.
Preparation For The Fight
Keep in mind that the squad that enters Ilos will be the squad that fights against Saren; players do not get an opportunity to change their squad after landing on Ilos. Bring the characters that Shepard fights with best on this mission.
Serious consideration should be given to Liara and Kaiden because they both have the Lift ability. Saren is immune to most biotics, including Singularity, but not Lift. Shepard should have either Barrier or Immunity (both are even better).
Sniper Rifles are particularly great for the squad against Saren because they pack a punch. They can also knock him around while suspended by Lift and will do additional damage based on where he lands and what he hits. Hit life pool is substantial, so these extra physical attacks are great.
Counter to common sense, Saren is considered synthetic life, as are the geth that enter the battlefield during this final encounter. Use mods that do extra damage against synthetics, along with the Sledgehammer and Hammerhead rounds for extra force.
Battling Saren
Saren himself behaves identically to a Geth Hopper. The Hopper is the unit that rapidly leaps everywhere and only stops to line up a sniper shot. Saren is basically this, but he can't be hacked, can't be controlled with Singularity, and has a deep pool of life.
When Saren stops to line up a shot, use Lift to interrupt him. This will both ruin his sniper shot, but also create a moment of weakness where he isn't darting around the map. If Lift is on cooldown, use Barrier or Immunity to absorb the blow.
Geth will slowly enter the room during the fight. Even on Insanity, these units aren't difficult and can largely be ignored or killed by squadmates. Their passive abilities will be more than enough to take care of the smaller enemy units. Keep the focus on Saren.
He will gain renewed life thanks to a cutscene partway through the fight but his tactics do not change. Keep up the pressure and he'll be down in no time. Congratulations on beating the first part of the trilogy!