Like the title says, how do you get them and what's the best stage to do it on?

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Annoy a troll...use facts and logic.

Nevermind I figured it out this morning. If anyone is still trying to get it here's how to do it.
Mission 15: Set up two spring traps on both sides of the pungie pit where the boss is. It should look like this, note that > means springs facing in the direction it should be pointed and X stands for pungie pit.

>>xxx xxx
This will cause the boss to go across the pungie pits 4 times for a total of 24 hits plus 1 for every spring trap he hits.
EDIT: There should be << on the right of the x's, don't know why they didn't show up.

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Annoy a troll...use facts and logic.

Thanks.

 

I'd already got the 15 and 20 hit achievements but that should help with the 25 hit one.

its much easier to do on 4.  Just set up the traps like so:
T=trap
|s|=spike

 T ---> T ----> |s||s||s||s||s||s| <--- T wait for the person to enter that trap and then place the second trap <---T

I believe there are 6 spikes on that stage and i believe you get a combo for entering the trap as well as hitting the spikes.  So imagine the enemy coming in on the right side of the spikes.  Walks into the first trap that is pointing left.  Gets pushed to the far left(x7), hits the first trap at the end of the spikes.  Since that trap is pointing right, he gets rubber-banded back across to the right (x14).  As this is happening you place the second trap down on the right side.  The enemy will once again hit this and be propelled back to the left (x21) and will once again hit the trap on the other side coming back to the right (x28).

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Zombies everywhere, zombies everywhere, zombies everywhere. They're coming for your brains and it's too late to change.