To earn the challenge entitled 'Skewer', you must kill 30 enemies with projectiles whilst in the games multiplayer mode, the Colosseum.
First things first, the wording of this achievement is a little misleading. Despite saying 'projectiles' and not pila, the ballistas do not count. Only the Pila you can throw.
This can be obtained on any map regardless of whether it is co-op or it is solo. You may choose to play through normally and pick up kills as you go. However, for those who are just wanting the challenge out of the way, here is the fastest method.
Load up Courtyard on solo. This method doesn't work in co-op due to load times, and time wasted searching for a partner. It took me approximately 10-15 minutes to earn using this method. This is to eliminate the luck part on waiting for a Balista to spawn with a decent Pilum rack.
When you start, 3-4 enemies will spawn ahead of you. There will also be two Pila in a corpse as you enter. This corpse will either be right as you enter on the right, or leaning against the column a bit in front of you. (NOTE - Sometimes this corpse doesn't spawn at all) Collect these two, and then take your first two kills. Don't rush with the quick-shot in case you miss, hold down LT to aim, wait for the reticule to go red, then RT to fire. In the first round, this is instant kills.
This will leave you with either one or two enemies left. Now, run to the opposite side of the arena, directly across from where you enter, you will find another corpse (NOTE - This corpse also sometimes doesn't spawn.) with 2 pila stuck in him. He is sat up against the wall on the right hand side. Note, he's on the far side of the wall so you will have to run over to that.
Use these two and claim your next 2 kills, which gives you 4 (or two if a corpse didn't spawn for you).
Now, idle and let the CPU kill you. The reason for this, is it's far quicker and skips loading times. Once dead, hit continue, hit new playlist and then hit courtyard. Now repeat this until you have 30.
If you are interested in saving seconds, you can equip gear so you can hold two Pila at one time, but it isn't needed. It would only save seconds per round.
Hope that helps and provides all the information needed, any questions, just ask.
** After earning this, and testing over multiple rounds, there was only one round where I failed to get at least one corpse spawn in one of the regular two locations. Sometimes, it may spawn in an out of the way location. In which case, my advice is to leave it, and not waste time searching.