I managed to kill the scarabs with only one shot, but I had to use a level 7 crossbow, a crow hat, a tier 3 arrow and the mutation for bow damage (level 2) (I think a tier 2 arrow can be used but I dont know), so it's not so hard to kill them (I know it requires a lot of things but their shell is used for the last upgrades in the game, therefore its ok for me). I liked the scarabs idea, but I have to agree that they are extremely rare considering 15 shells are needed to fully upgrade a weapon, if less were needed I believe it would be ok.