The summons strenght can kinda be assumed by its needed stats, obviously a summon1 will be weaker than summon10 ;)

LV1
-- 1 Slot per skill - living 10 turns - cooldown 4 turns - use 1AP --
Summon Fire Slug (Fire Element -> Summon1 Fire1) << Starter
Summon Water Slug (Water Element -> Summon1 Water1) << Starter
Summon Poison Slug (Poison Element -> Summon1 Earth1) << Starter
Summon Source Hound (Warrior -> Summon1 Warfare1) << Starter
Summon Source Hound (Ranged) (Ranger -> Summon1 Ranger1) << Starter

LV2
-- 1 Slot per skill - living 10 turns - cooldown 4 turns - use 2AP --
Summon Beetle (Poison Element -> Summon2 Earth2)
Summon Fire Beetle (Fire Element -> Summon2 Fire2)
Summon Frog (Water Element -> Summon2 Water2)
Summon Gheist (Assassin -> Summon2 Scoundrel2)
Summon Void Deer (Fighter -> Summon2 Warfare2)
Summon Fire Automaton (Fire Element -> Summon2 Fire2)
Summon Air Automaton (Air Element -> Summon2 Air2)
Summon Poisonous Trunk (Warrior -> Summon2 Warfare2)
Summon Cat (Assassin -> Summon2 Scoundrel1)
Summon Bear (Warrior -> Summon2 Warfare2)
Summon Turtle (Warrior -> Summon2 Warfare1)
Summon Bone Crusher (Skeletal Tank -> Summon2 Necromancer2)

LV3
-- 2 Slots per skill - living 6 turns - cooldown 6 turns - use 3AP --
Summon Giant Crab (Strong Water Element -> Summon5 Water2 -> Uses 3AP)
Summon Spider (Strong Poison Element -> Summon5 Earth2 -> 2AP)
Summon Clay Soldier (Earth/Warrior -> Summon3 Earth3)
Summon Skeleton Mage (Strong Dark Element -> Summon2 Necromancer5)
Summon Skeleton Archer (Archery -> Summon3 Ranger1)

LV4
-- 3 Slots per skill - living 3 turns - cooldown 10 turns - use 4AP --
Summon Fire Dragon (Fire Boss -> Summon10 Fire10)
Summon Frost Dragon (Ice Boss -> Summon10 Water10)

Special Summons
Create Source Puppet (Multielemental -> Summon5 -> 2AP 1SP because lore)
Summon Lucky Bunny (Lucky Charm -> Summon3 -> 3AP -> May die in 1 hit, has Aura's -> 2Slots)