Some nice combos

COMBO 1 :

Game Rule Requirements...
Up Close & Personal: Only Monsters with Melee attack may be used in Battles

These 2 cards combined even at Level 1 make one of the most powerful combos when only Cards with Melee are allowed to be used since you will know for sure that all the attacks from your opponent his cards will go -2 which is huge. The Dragon/Fire Summoner on top has some extremely powerful melee attack cards for both higher & lower mana Fights that can be used from any position.

Required cards Type
Camila Sungazer Legendary
Molten Ogre Common

COMBO 2 :

Game Rule Requirements...
Target Practice: All Ranged and Magic attack monsters have the Snipe ability.

When ranged and magic attack monsters directly have snipe, the Blast ability is extremely powerful as it will attack 3 different monsters. The Water summoner has 2 powerful cards that are quite cheap that have this ability. (The Fire Splinter also has many but most are more expensive and less powerful). Having both these cards at a higher level increases on the damage that is being done.

Required cards Type
Ruler Of The Seas
Pirate Archer Rare

COMBO 3 :
This combo is quite fun and works with different game rules as long as there is at least 25 Mana available to select cards.

Basically, the Taunt ability of The Kraken forces the opponents to attack that cards and since it has so much health combined with a triple heal from the Albatros (Level 6) | Tortisian Chief (Level 4) and The crustacean king it is impossible to kill unless the opponent has the Affliction ability.

A similar strategy could be done with the Life splinter using the Shieldbearer combined with the Divine Healer, The Angel of Light, and the Tortisian Chief.
This strategy does require higher-level cards that are more expensive.

Required cards Type
The Kraken Legendary
Crustacean King Rare
Albatross L6 Common
Tortisian Chief Epic