For defense battles - Do not leave your base undefended, otherwise your opponent will just walk into it with his soldiers unopposed. Deploying proper cards at the right time is very important while playing defense battles. Using buff cards like Iron Maiden at the right moment can completely change the tide of the match.
Online matches are mostly won or lost by how well you use your cards to counter the opponent's strategy. Pay attention to their hero skills and what cards they have played so far in order to be able to predict their next attack strategy, while also deploying appropriate cards of your own . Plan ahead for winning these turn-based battles.