THE FINALS

This one is easier with a friend playing Medium healer and you playing Heavy. Both of you should take pyro grenades. Communicate with your friend and set yourself on fire when enemies are nearby. Your friend should keep you safe with the healing beam while you take the kills.
I think Bank It is more convenient for this achievement but of course it is achievable in any mode you prefer.

Also achievable during the Alien Invasion event. You can bounce off the laser beams of the UFO.

https://youtu.be/zpZs8nt0gMg

Multiple ways you can do this one, I found the easiest to be with Light, cloaking device and the stun gun. You can use any weapon you prefer, I found the XP-54 to be the most appropriate thanks to its accuracy.
Find an isolated enemy, stun them and get them to low health. Then get up to speed and slide. Aim for the head. You can earn the achievement with hip fire as well.
Bank It should be the best mode for this one, but of course it is achievable on all modes.

https://youtu.be/jZAhnSmx1Xo

Light with cloaking device and stun gun should be the best for this one as well. Try to camp around areas with a lot of carriables within your reach.
Ordinary objects like flowerpots, chairs, tables etc are more preferable than gas cans since they are easier to hit and can't kill your target as easily.
Wait for an isolated enemy, stun them and try your best to hit the carriables.
This achievement requires patience and luck but shouldn't be too much of a problem. Bank It is probably more suitable for this one.

https://youtu.be/PxCYea9-ddo