For a magic build I recommend getting the circle of life skill (gives one umami when killing an enemy with an umami based attack) and using the any shell with shards (shot glass and mason jar are best cause high def). With this you can pull as many enemies as you want, use shatds when one is about to hit and if thats not enought do it until they are all dead. With this you badically have unlimited umami and a shell skill that makes you invulnerable whenever its active. If you lose your shards shell, you can use the crab adaptation in a similar way once its fully upgraded to only cost 1. For stowaways I mostly used the ones that give you more scrap but other options are the one that give 33% more umami per attacks so it only take 3 per charge instead of 4 (useful for bosse where you cant abuse circle of life). Other optikns are the ones that give 20/40/60% shell spell damage (doesn't work if using the crab adaptation unfortunately) and the fish ones that attack enemies if you find yourself not instakilling most enemies, if the fish finish them off you still get the umami. Later in the game, the spirit conch you get from the shell adaptation is also really good once fully upgraded as it lets you shoot enemies from afar.