The undertray and diffuser are parts on our vehicles that need to come off a lot; whether that be for making repairs or just to check for leaks, tears etc. However, the stock undertray bolts can make this a real pain.
The stock undertray bolts are button-head alleys which can strip. The problem usually stems from improper use. An allen head bolt starts to strip from overtighting them, or not putting the allen key all the way inside the head of the allen bolt. (This goes for all allen heads. Not just the undertray and diffusers. Allens are used alot on these cars, so pay attention that your key is all the way in when removing and be careful not to over-tighten.) Conversely, if you do not remove your undertray much, the bolts start to seize inside the hole, due to rust.
One of the easiest ways to get them out, without drilling, is to get a cutoff wheel and cut a straight line through the head of the bolt. Use a punch and hammer, and tap the bolt in the direction to loosen it. You can try to use heat, but the undertrays are very sensitive to torches due to the material and the fact that it's so thin.
The easiest way to avoid all of this: replace the button heads with a standard hex head and add a little Anti-seize. You can purchase these bolts at any hardware store (M8-1.25x25 for both the undertray and diffuser). The only way to strip these bolts is by using the wrong size tool. Even if these do seize in the hole, they will come out with a standard wrench.
Highlighted in red are two of the bolts and where they should be replaced.
Here's a chart that is a great guide for torque specs.