There are quite a few ways:

You can try to mance him on a moth, a glider around him, going behind his back, but there is a chance that he will grab you with his ass anyway. You can simply swim into a small cave or crevice where he won’t reach you, which is logical. You can use the bug, if you are interested, there is a bug that if a builder builds something at least 1% in front of his face, he will fly far away no matter what. You can freeze it in stasis with a rifle and sail to a safe distance, or stun it with the perimeter protection of the moth, which is the safest option, but keep in mind that it can grab small vehicles through stasis and cause damage to the character. Well, if you are a maniac or he has already fed you up, then you can just kill him. There are only 3 options: freeze him in stasis with a rifle and beat him with a knife until he dies (125 blows with a thermal blade, 250 blows with a knife). He hooks the crab suit onto it with his hand and drills until he dies. And the longest and most complete one, as for me, is to shoot torpedoes with gas follicles from a moth and wait until he dies from it.
And also just a mini interesting piece of information, if you kill the reaper in the crash zone, he will not respawn after 2 game days than in other locations. And also an addition from the comments and how the easiest and fastest way is to freeze the reaper with a stasis rifle and throw as many gas follicles in front of his face (poisonous death is guaranteed).

As a result, the reaper is not such a terrible leviathan as it seems at first glance, because you see quite a lot of methods and methods to counter this toad. Well, thanks to everyone who has read up to this point, I eat pilaf and drink kvass.