Solo monster. To summon a solo monster go to the "Monsters" page and click the "Summon Solo" tab. You will see a list of available monsters. Choose one and click "Attack".
To summon a multiplayer monster, click on the "Monsters" button in the main menu. You will see the list of monsters that are available to you. Choose one and click "Attack". The battle screen will appear. You can't summon more than one monster of the same type. For example, you can't summon 2 Trolls at the same time. The second one can be summoned 1 hour after you defeat the first one. If you don't want to wait, you can revive the monster using gems. Don't forget to collect the reward after the end of the battle. If you don't collect it, it will disappear.
There are requirements you need to fulfill before you can summon some monsters. For example, a certain amount of Arena wins, a certain item or a potion in your inventory. If a monster has any requirements, they are displayed in the monster list. Please note that any boosts or potions that are present in the requirements will be spent to summon the monster.
Altar monsters. To summon this type of monsters you need to donate a number of different items to an Altar. Each altar has its own item requirements. As soon as these requirements are met, the monster will be summoned automatically. There are no "per player" requirements. If you donate at least 1 item, you will be in the fighter list. To donate an item, specify the amount and click "Donate". If you don't donate, you won't be able to fight the monster.
The amount of monster's health depends on the amount of fighters which took part in the summoning. More fighters means more monster's health and the other way around. After the monster is defeated, it will be unavailable for some time. After 12 hours you will be able to donate items and summon the monster again. You can summon Bloodlord through the Mansion, Mountain Troll through the Troll Cave, Poisonous Hydra through the Foul Pit, Minotaur Captain through the Maze and Sea Beast through the Cliff.