Lewyn +atk-res or +spd-hp

Go spd since his tome works as a desperation 4 so you can double before your opponents get a chance to hit back

Definitely speed as his tome gives him desperation if he is above 50%, meaning he needs to be able to double in order for that to mean anything.

Assuming you buff both stats at +10 he will have 51 speed and 54 speed with an without the bane. This gives up 6 damage andI think most people won’t reach that kind of speed. Especially if you have debuffs