Get the Summon spell at any level, then go somewhere where there is a high level monster or the monster you want for your summon. Use your melee weapon and hit it until it becomes unconscious (it will sway, and a little star will fly around its head). Then, summon your creature. Hit the monster with your fist, not your weapon, and allow your summoned creature finish it off. If the monster kills your summon, re-summon it until it kills the monster. When the monster falls, it will glow blue, and your summon creature will become the monster that just died.