Orcs Must Die achievements for all you achievement hunters. Here is the nitty gritty on the more difficult ones in the game.
Ogre Bisque – Kill an Ogre in a Lava Pit or an Acid Pit
Simply get to the point where you unlock the weavers. Then, choose a level with plenty of opportunities to use a push trap into some lava or acid. Tower is a good map for this achievement. Upon starting, make sure to choose the Steel Weaver. The last upgrade on the bottom right is Overcoiled Springs, which allows your push traps to actually have an effect on ogres.
Pow! Pow! – Get 50 kills with explosive barrels on one level
This achievement can be tricky to grab. Make sure to utilize tar traps to slow enemies down, and the wind belt to push groups back into one another, allowing you to get large groups of orcs at one time. Then blow them up with explosive barrels.
Who Wants Pancakes? – Get 50 kills with the rolling log in one level
Load the Tower map, equip a wind belt and follow this video:
No Traps for You! – Win an act 2 or 3 level without using any traps
Seems to be easiest on “Chokepoint” on Apprentice difficulty. Load up your bar with both the guardians and all of the weapons. Use the Steel Weaver to upgrade the guardians. Then just drop lots of archers up on the balconies, and put a few paladins at the front of the bridge. Then just use your favorite weapons to help out the guardians. Most people like the lightning ring, but I personally prefer the Alchemist satchel for crowd control. From there it's a snoozefest. You might let a few Kobold runners through on the early waves, but don't worry: the achievement only says you need to beat a level, not perfect it.
In your Face! – Get 100 acid pot kills in one level
Here are a couple of maps to get this achieve. First map is The Corners. There are 2 acid pots. Put some tar traps under it and use a wind belt to bunch them up. When they under pots shoot it. You should get the achievement in 4-5 waves.
Second map is the DLC level “Double Trouble”. Stand in one of the entry rooms and wait until the orcs get halfway through the room. Then shoot all 6 of the acid pots, and shoot them again once they refresh, and it should kill almost all the orcs in the room, even on nightmare difficulty. Once you set up your standard killzones in the stairwell hallways, occasionally pop into one of the entry rooms as it's filling up with orcs, and shoot all the acid pots down again. You should get the achievement in 5-6 waves.
Who Wants Some?! – Get a 40 kill streak
A great Level for unlocking this Achievement is Level 22 “The Squeeze” (Mission 4 of 6 at Act III), and you just need to reach Wave #4.
The Equipment you will need is the “Windbelt” and the “Tar Trap”, and any Weapon which can be used for killing flying Targets. It's a good idea to play this Level at Difficulty “Warmage”, because it's necessary to place the Tar-Traps at the upper Floor before Wave #4 starts.
The Orcs will be launched in 4 Groups each with 15 Orcs. Before the Wave starts, you should place 9 Tar Traps in the Square in the middle of the upper Floor, because the Orcs need to be slowed down. And then press “G” for starting the Wave.
As soon as possible, you should kill the 5 Bats (but without using Mana!). Then use the Windbelt to hold the Orcs near the starting point, until all 60 Orcs are launched. When all Orcs are “in position”, turn around and shoot at the rolling log. If everything is right, you will get a Kill Streak of 50 or higher.
Great Balls of Fire! – Get 300 fireball kills in one level
Play Chokepoint on Apprentice difficulty. Equip tar traps, flame bracers, and mana well. Use the Elemental Weaver and get Mana Conservation and Firestorm.
The bridge is exactly wide enough for a full fireball to hit, so you don't need barricades, which are expensive. Cover the bridge in tar traps, first in horizontal rows. When you have about 6 (two rows) then build a mana well to help you recharge the bracers faster.
As you get money, work toward building 15 tar traps and 4 mana wells. This will allow you to constantly throw fireballs and kill everything. If a few orcs get across the tar, you can lure them around at close range until your last fireballs charge up.
If you kill every single enemy with fireballs, you'll get the achievement on round 7.
Lights Out! – Get 20 chandelier kills in one level
The best level to do this on is “The Corner”. Just bunch enemies up under the chandeliers with tar traps, then shoot the chandeliers. It's relatively easy to do, just make sure you get the timing right.