原文A creature you control is attacking alone if it's the only creature that's currently attacking (including creatures controlled by your teammates, if applicable) and you control it. For example, if you attack with multiple creatures and all but one of them are removed from combat, you can activate Crowd of True Believers's ability targeting the remaining attacking creature.
原文If there are multiple creatures attacking, it doesn't matter who or what any of them are attacking. If you control a creature attacking one opponent and another creature attacking a second opponent (or even a planeswalker a battle controlled by the first opponent), neither attacking creature is attacking alone.
原文If the target creature is an illegal target as Crowd of True Believers's ability tries to resolve, it won't resolve and none of its effects will happen. You won't gain life.