Choosing a Summon
The first thing to consider when choosing a summon, is the purpose of the summon, whether shooter or blocker (see last week's tip).
When summoning a blocker, choose a creature that has high defense (the damage they can do is only a minor consideration). If possible, choose a summon that is immune to your target creature's attacks. For example, a Demon Skeleton is a good summon for blocking Dragons
When summoning a shooter, choose a creature that has strong distance attacks. The best summoned shooters will try to keep a distance from your target, so that they do not get attacked. If fighting creatures with immunities, make sure your summon does not use that kind of attack (do not summon a Fire Devil to fight Orc Leaders). For example, Fire Devils and Fire Elementals are good for fighting Scarabs and Minotaurs.