The Unit needs to be alive when Tactic Cards are revealed to count towards Strength bonuses. In your example, the Paladin would lose his +1 Strength if the Serf were killed before the Tactic Cards were played. Furthermore, the Level 3 Paladin can gain an extra 2 VP for winning a battle with a Serf. In this case, the Serf would need to survive the entire battle (not be sacrificed) in order for the extra VP to kick in.