Sera’s attacks are key to winning with Vanguards. Between Citadel Knights and carefully poking low-life units, you should trigger her ability as much as possible. In Renewed Hope turns, you can generally attack with her freely and then protect her from any possible counter.
Her endgame is scary as she can produce an infinite supply of Citadel Knights while she pokes with her ability, out-valuing every opponent summoner in the game. Sera’s life is precious, and with careful positioning, you can effectively convert her life into magic and units. She is the core of the Vanguards’ economic game, trying to force the opponent to place units in her range for her to gain value. Smart opponents will avoid this but will then have trouble not giving up territory. Forcing the opponent to retreat from Sera’s threat is a big part of her game.