First, there are three waves of enemies. The first is four Strikers - the enemies with time-blocking harnesses. The first one will warp up right up to you, which you can stop with a time shield and then blast point blank while he's suspended. Then you can run down the platform with a time rush to the other one standing straight ahead and kill with a melee attack. Two down, only two left. Just time dodge when you see the red around you, and as someone stated earlier, when you see red and are confused whether it's taking damage or something else, just do a quick rotate and look for the orange orb. Then dodge. There's an audible queue too - Jack will say "gotta run" or "no no no!". That's when you know to run. It doesn't help the level design has so many obstacles to stop from a clean dash, so you need to know where your open spaces are. Also, leveling up your time shield so that it heals helps quite a bit.
Then, Serene will unload a big blast, but it starts right where you are standing. So if you kill the last enemy by the entrance to the poolhouse, just run to the back of the room when he's blasting. It helps that there's a backpack in the back to stock up on ammo anyway. After the blast he becomes vulnerable, so you can take the first shot at him after the blast. Serene is sitting above the pool at this time, but if you wait too long you'll miss your chance and the next wave will begin. Also, the right side of the pool house has far less obstacles in your way than the left, so it's a straight run to the back of the room. Critical to escape the blast, and good to know for the end of the fight.
Then comes the second wave, which consists of four or so Monarch security guards armed with grenades and a big armored tank (not a Juggernaut) with a chaingun that makes things difficult. To make this an easier, fair fight, once you shoot Serene turn to face the entrance to the poolhouse. Having a good powerful automatic weapon here helps, like the semi-automatic rifle (don't remember the official name, but it has a capacity of 20 shells). You can do one-shot kills to Monarch security with this, so when the wave starts, you'll get a chance to take out two of the four before the tank comes out. That really helps because that's less the number of guards who will rain grenades on you, and one less thing to worry about when taking out the tank. Don't waste your time blast on the entrance - it wouldn't work for me. Once you take care of the security guards, you'll then have to deal with the next wave of enemies, another group of Strikers. Since they warp in close to you, just stay clear of the tank and take these out post haste. Then that leaves the tank and the blasts coming from Serene. I'd advise you use your time blast here on the tank - just fire the blast and follow up with a barrage of bullets - and stay far away from him. The left side of the poolhouse is optimal here because of all the obstacles which can be used for cover. Just keep using time vision to know where he is, and keep him on the left because he's slow and will also get caught up in the obstacles, restricting his movement and leaving him open for some direct hits.
Once you get him, Serene will start to send some blasts your way. They start back to front, and then reverse endlessly, leaving just a few seconds in between for a clear shot. Serene is vulnerable during this time, so wait for the blast, run towards the front, watch for the front blast, run to the back. Dispense some ammo in Serene's direction in the gaps. And don't get caught up avoiding the blasts - this is where again it's important to stay on the right side of the room, because there's little on the path to get in your way. It only takes a couple of shots to end Serene and the boss fight.