Among Fusion Monsters we find the highest ATK monsters available in Duel Links. The most popular ones are "Meteor B. Dragon" and "Twin-Headed Thunder Dragon", as they provide really good ATK/DEF value. They can easily win games by themselves in any Auto-duel against Standard Duelists. Players need either "Polymerization" or "Fusion Gate" to summon one of them. Having multiple copies of those 2 spell cards is the key to building a Fusion themed deck.
To build a fusion deck to win Auto Duel, players often run 5 copies of the aforementioned spell cards in their decks, together with 15 Monster Cards including correct fusion materials and substitutes, such as Versago the Destroyer and "Goddess with the Third Eye". Doing so you can rely on the “Balance” Character skill, achieving a 100% chance of drawing fusion spells on the very first turn.