I'm going to Assume that your referring shin grimm and if that's the case your best chance of increasing your damage is to get better str stones,infuse his weapon, and forge to the highest level possible and to use your 2nd skill until your able to upgrade grimm then he will automatically help your team do 15% more damage then you can drop the 2nd skill.
Also grimms anti heal doesn't work on if a sajin is fully modded due to his break immune mod which negates grimms anti heal effect completely.