This has to be their A+ Support. If only we could've gotten S Supports for pairings other than Byleth.
Here's how I'd do a Sylvain/Ingrid one:
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Sylvain is in the middle of a battle when the enemy commander knocks him down. Before said commander can finish him off, Ingrid swoops in and kills him, effectively ending the battle. Sylvain thanks Ingrid for the save, to which Ingrid sighs and says "What would you do without me, Sylvain?" Sylvain, who is on his knees, takes the opportunity to pull out a ring and tell her that he really can't live without her. Ingrid is caught completely off guard, as she thought Sylvain just wanted to be friends. Sylvain says he can't just see her as a friend anymore, and that since she's done so much for him, he wants to make her happy for the rest of her life. The two are then given a load of encouragement by their troops, who think the two are a perfect match. Ingrid accepts Sylvain's proposal, prompting him to kiss her in front of their troops. The troops cheer for this, but Ingrid is of a "Not in front of everyone, you dummy!" attitude.
This would've been so perfect. Too bad Nintendo's line of thinking was that the S Supports had to be special for Byleth.