There really isn't a lot of places to get this, in fact I only found one that was fool proof. In Act 3 Chapter 12: The Imperfect Storm. You will have to protect DNA carts. During the 4th one, the enemies will keep respawning every time you activate the computer console. Abuse this. Keep pressing it, then letting the cart get destroyed. Doing it 3-4 times will create a ton of enemies (you won't be able to see some of them). Now stand in front of the cart, hold to do an area of effect attack as long as you purchased it. Keep doing it until you get the achievement. It's extremely easy.
after you reverse that magnet area and it shoots you back up to the area where you have to connect all the DNA carts, tons of the half mutated guys show up. I got 227 before it switched to the other spiderman.
#3 is right. When the 4 DNA Carts are charging, just activate a decoy (hit LT), charge a Roundhouse Spin (Hold X to charge) and deactivate the decoy (tap LT again). Repeat this and you shouldn't have any issues. 297 combo was on my first attempt here.