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Here again, it depends on who you're playing as; for a Knight in the Single Adventure, Axel is the best choice, as he levels up his defense. You can also level up his luck (level 28-29). If Axel is at that level, he'll be able to defeat enemies much higher than level 3. (With my character, I was able to defeat up to 90 peasants at level 40). And it's better to take 2 units instead of 3, especially if you're focusing on soldier's luck. It's much better when peasants don't just stand there, but also push back and have better defense.