Summon Mastery doesn't affect the creature, just your summons. The skill is at its best when you max it out in concordance with a maxed out summon.

You're playing as an archer, so a good tactic is to go for the Demon (melee summon) and a creature with limbs that give and/or increase its melee skill/prowess. This way you'll have two allies attacking at close quarters that distract and damage the enemy while you stand back and pound them with arrows.

I am in blood
Stepp'd in so far, that, should I wade no more,
Returning were as tedious as go o'er.