I like the critical hit idea, and I also like the helping another player idea. Here are five versions:
1. True Strike - Critical
divination cantrip
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Casting Time: 1 action
Range: Sight
Components: S
Duration: 1 round
You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target’s defenses. You have advantage on your next attack roll against that target before the end of your next turn, and if it hits, the hit is a critical hit.
2. True Strike - Critical, Any Helper
divination cantrip
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Casting Time: 1 action
Range: Sight
Components: S
Duration: 1 round
You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target’s defenses. The next attack against the target before the end of your next turn has advantage, and if it hits, the hit is a critical hit.
3. True Strike - Critical, Specific Helper
divination cantrip
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Casting Time: 1 action
Range: Sight
Components: S
Duration: 1 round
You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target’s defenses. Choose one creature of your choice. That creature's next attack against the target has advantage, and if it hits, the hit is a critical hit.
4. True Strike - Specific Helper
divination cantrip
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Casting Time: 1 action
Range: Sight
Components: S
Duration: 1 round
You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target’s defenses. Choose one creature of your choice. That creature's next attack against the target has advantage.
5. True Strike - Any Helper
divination cantrip
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Casting Time: 1 action
Range: Sight
Components: S
Duration: 1 round
You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target’s defenses. The next attack against the target has advantage.