Knight Squad

You need to get the horse powerup twice--picking up weapon or speed powerups won't cancel the horse, but DON'T pick up the bull powerup! That will make you explode..
Once you get the horse powerup twice, get a kill for the achievement! The achievement pops at the end of the match, so don't worry if you don't get it right away.

Solo Method
By far the easiest method is doing this on Gladiator mode, with the map Deadly Corridor. I recommend 5 minutes.
I waited on the right side waiting for the right powerups and was never in danger of dying. You just have to wait for the right powerups.
What's even better is you have unlimited exploding arrow powerups from the frequent spawns of the two bow/arrow powerups.
Make sure to use up all the powerups that you can to get the spawns you want faster..

I tried this in other modes, and they were frustrating and I kept dying or having the 'reset' powerup activated.

***Update December 10, 0218--2 controller method just worked for someone so it still does work!
**Update August 30, 2018--A LOT of people are having trouble with the 2 controller method. It might not be very reliable at this time.
*Note: Update 9/2/17 Apparently you must have a 2nd controller logged in for the Two Controller method to work!**
Two Controller Method(Thanks to I8ITackyticsI8I!)
You do NOT need to have a second gamertag logged in. This is what I did.
Two controllers, last man standing, 1 round on watch out for death. Run around to get 2 horse pickups and level 2 bow. One shot will kill but even after the match the achievement won't have unlocked. Confirm both controllers and you will go back to select battlefield. On the second controller double tap the guide button, go up and go to add and manage under all the user accounts on the xbox. The achievement will then pop at this point without logging in!

For this achievement you must kill an enemy while riding a unicorn. To get a unicorn you have to get the horse power-up twice. This achievement wouldn't unlock for me with a second controller, so I did it against bots. Here is a video of it being done. (Update: If you're having trouble on Juggernaut mode try it on CTF or Gladiator instead)

The achievement is to get a kill on a unicorn (level 2 horse). Using a minion to get the kill does not count. This achievement can be unlocked against a 2nd controller and unlocks at the end of the match.

In the video below, I show the entire match in which I got the achievement. However, I get the unicorn at the 3:40 mark and proceed to finish the match at that point.

For all those who have trouble with this second controller login.
I did the solutions above and did not popped for me, then log in to second controller with guest account did not work. you actually need to go in the menu of pick a knight, and change his name with the Y button, to your guest profile and your name of the knight must be ....(1) Try again and it popped for me after the match!

You have to take 2 Horse bonuses in a row to get a unicorn, then kill a knight. Here is a method to unlock this achievement with a second controller. Start a game with these settings:
- Difficulty Bots: Disabled
- Game mode: Capture the flag
- Time: 5:00
- Level: First Flag