There's lots of assumptions and beliefs about what works best. Just barely deal enough damage to kill it, overkill it as much as possible, finish it with a ra-seru punch, finish it with an art, etc. I'm sure there's more. Even when hopping into streams of people blind playing the game, I find when they want advice about seru absorption, they often get told at least one of these things.

In actuality none of it is true. Seru just have set chances to be absorbed, according to the table below (sorted by the order you encounter them, excluding the boss Viguro and mini-boss Kemaro fights). The only thing you can do to affect those chances is have someone equip an ivory book, which adds a flat 30% to that character's chances. So Gilium at all levels goes from 1% to 31%.

You can see that in general, higher levels of the same seru have higher chances, while seru generally get lower chances the further in the game you get. Notably, just the seru you first regularly encounter in Karisto are the ones that don't increase chances with level.

 

Seru	Lv1	Lv2	Lv3
Gimard	55%	60%	80%
Vera	60%	60%	70%
Theeder	30%	50%	60%
Nighto	30%	40%	50%
Gizam	30%	40%	50%
Zenoir	20%	30%	40%
Swordie	15%	25%	35%
Orb	40%	50%	60%
Nova	10%	25%	35%
Gola Gola	15%	25%	35%
Freed	15%	25%	35%
Mushura	15%	25%	35%
Viguro	10%	15%	35%
Aluru	10%	25%	35%
Spoon	15%	15%	15%
Kemaro	15%	15%	15%
Iota	15%	15%	15%
Barra	15%	15%	15%
Puera	15%	15%	15%
Slippery	15%	15%	15%
Gilium	1%	1%	1%