Since Ubisoft changed the trophy requirement, my guide is no longer valid.
The new requirements are much less tedious, just fire through mattresses/wood walls when playing online against players. Overtime you will get this.
There is a great spot to boost this... getting anywhere from 5 - 20 penetration kills.
1. Go to Options:
2. Matchmaking Preferences
(In Terrorist Hunt Section)
3. Turn off all modes except Defend Hostage
4. Turn off all maps except Club House
Load Terrorist Hunt - Lone Wolf
Now pick your favourite defender. Mine was Kapkan (trip wires are a blessing for the room across the hall)
Now you will notice a matress in the middle of the room... use that as the barrier between you and the terrorists. You can shoot through it and get penetration kills. Be aware they can shoot you as well... It's still a grind but that took me only about a hour and a half to grind out 170 penetration kills, and I haven't seen a better way to do it as of yet.
Side note: I also completed the Terro Hunter here as well, because after each match, of four waves, I would be anywhere from 32 - 37 kills. And be aware, if you are looking at the tracking on the site http://rainbow6.ubi.com/siege/en-CA/community/stats.aspx
know that the terrorist hunt stats include Situation Maps. So anything you did in situations will add to those amounts but not count for the trophy. My trophy popped at 238 penetration kills. My terro hunter trophy popped at 2805. So I had 305 kills and 38 of those were penetration kills in Situations.
Hope that helps.
**just discovered that the requirements are different for this trophy since Ubisoft changed several trophies, so this method is unfortunately invalid unless you are working with the non patched version- no reason not to, since the requirement is much smaller and easier**