Combining cards grants combo points, and creates (usually) stronger cards from two Combo Cards. Combining higher rarity cards usually makes higher rarity cards and increases the amounts gained per level for the cards created. When combining cards, the average level of the two cards, rounded up, is used to determine the level or the resulting card. A level 1 card combined with a level 2 or level 3 card will give a level 2 result.
When combining the cards during a duel, the GENERAL rule is their attack and defense goes up one less per level than if you were upgrading your cards for gold and diamond quality cards. For example, a level two upgraded Gladiator would usually have 14 attack and 7 defense, but when combining a level two knight and trident during a duel, the Gladiator's attack will be 13 and defense, 6. A level three combination will have its attack be 15 and defense, 8 rather than an upgraded level three Gladiator with 17 attack and 10 defense. Arena opponents also follow these rules in matches, even though they are computer controlled.
While the above rule is typically correct, there are numerous exceptions. While the exact mechanism is not explicitly stated, it appears to be the quality of cards used in the combinations determining the stat increase per level. When you make the Gladiator as above, you will see +2 per level as described. If you make a Skeletal Dragon (Undead(B) + Dragon(B)) though, you will only receive +1 per level. For silver combinations, when making the combinations of World Tree ( Tree (B) + Water(B) ) you receive +1 per level as expected, but ANY silver combination with Superhero(S) including Rockslide, Aquatic Defender, The Incinerator, Vampire Hunter, etc increase at +2 per level. Likewise with Bronze, which are typically +1 per level, Giant Sword (Sword(S)+Metal(B)) gets +2 instead. Using a Gold combo card causes the combination to increase by +3 per level, regardless of quality. Thor's Hammer (9/9 base) created by Energy+Hammer has Lv 3 stats of 15/15, or +3 and Earthquake (8/0 base) created by Energy+Earth has Lv 3 stats of 14/6, also +3.
This means that Final Cards with multiple combinations eg Clown (Demon(B)+Rainbow(B) or Human(S)+Rainbow(B)), get different stats based on the combination used. This has been verified with Phoenix (1) 11/4 using Ash(3)+Fire(4) => Phoenix (4) 14/7 or +1/level and Ash(2) + Magic(2) => Phoenix (2) 13/6 or +2/level. Making Thor's Hammer (9/9) with Hammer+Magic creates a 13/13 level 3 card(+2), while using Energy+Hammer makes a 15/15 level 3 card (+3).
TLDR: If the combo uses at least one gold card it is +3 per level, at least one silver +2, only bronze +1
A non-exhaustive list
- Diamond (not possible, all require at least one gold card)
- Gold (+1) Skeletal Dragon (Dragon+Undead), Hydra (Dragon+Dragon) Poison Dragon (Poison+Dragon) Gorezilla (Dragon+Radiation) Unicorn (Rainbow+Horse) Supernova (Sun+Sun)
- Gold (+3) Thors' Hammer (Energy+Hammer) Bear Magus (Bear+Magic) Polar Bear (Bear+Water) Fireworks (Energy+Ash/Fire) Super Bat (Science+Bat) Blacksmith (Hammer+Science)
- Silver (+2) Aquatic Defender (SB), Treant (SB), Vampire Hunter (SB), Rockslide (SB), The Incinerator (SB), Morning Star (SB), Red Knight (SB), Blue Knight (SB), Green Knight (SB), Dwarf (SB), Medusa (SB) Phoenix (SB only)
- Bronze (+2) Giant Sword (Sword+Metal) Trident (Sword+Water) Hammer (Sword+Earth) Undead (Human+Undead) Holy Water (Magic+Water) Golem (Magic+Earth) Ash(Human+Dragon)
- Bronze (+3) Death (Time+Human) Ghost (Energy+Death) Earthquake (Energy+Earth) Car (Energy+Horse) Radiation (Energy+Poison) Bomb (Science+Radiation)
Post if these combos do not have the correct modifier
When playing computer opponents in events or regular levels, each level of cards adds only one point per level regardless of quality. This means the computer lv 5 Gold combo (+4/+4) will be the same as the players identical lv 3 combo (+4/+4).