I handle Astarte by dying.
I cannot read her moves when I'm zoomed out so I always get hit by the heart arrows. I know I should be using the mist form when that happens with Al.
I've not beaten the Reaper in multiplayer or single player, as I only very recently unlocked his stage. I did reach him but he destroyed me.
Probably shouldn't zoom out when fighting Astarte xD Zoom in is the first thing you should do before fighting her especially. Best way to fight her I find is staying behind her as much as possible. So attack about 2 - 3 times then jump over her, rinse and repeat till she dies. Once she is about to use temptation, there's a small gap where the hearts can't hit and you need only double jump over it (use the mist form as well to keep yourself in the air for a few seconds longer).
How-to: Farm Chapter 7 for Miser RingsRight at around 2:10, if you pause there you can see where they will come from and the double jump ought to clear it relatively easy. But when she's against a wall and you're infront of her, you'll have no choice but to use the mist form to dodge it (or have a friend hold the door xD). Astarte took me some time to learn her pattern well enough, and now I can fight her confidently (well, except when other players are spamming their laser attacks, cus then I can't see Astarte for me to read her moves.
Her green triangles of death can be dodged by ducking to my understanding, and her tornado just pick up on her motion before she does it then jump over it (or if you feel risky, you can crouch infront of her and attack at punch distance. That dodges the triangles and the tornados, but not her close range attack or heart arrows).
The reaper's patterns can catch me off guard at times as well, even after seeing his cues of his attacks. Just try to pick up on the tells of his attack, then you should be able to relatively dodge his attacks good enough.