Is there a way to improve Explosion hits?

I’m making the Thunder Spears from the Attack on Titan anime. All is fine, it deals damage, it fires properly. However, I made it so if it hits a part located in the nape (neck) of player titans, it will deal a lot of damage and play a special animation. If it hits anything else, it just deals moderate damage.

The thunder spear hits the nape of the player titan and does what it’s supposed to do. However, sometimes, if you hit the front top of the chest of the player titan, it sometimes hits the nape part.

The thunder spear exploding on the nape of a player titan:

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The thunder spear hitting the nape of a player titan despite it being on the chest:

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So how can I improve this? I don’t want to decrease the explosion radius (because i want it to be used against human crowds aswell) Is there an alternative to Explosion.Hit?