Iro Hero

In level 1, there's gonna be 4 columns of 6 enemies that come at you from left to right. Start shooting them to kill them as quickly as possible for 24 hit combo.

Full achievement walkthrough here:

There is a point halfway through the first place where enemies are stacked in columns, and they appear left to right. If you hit the left column, then the next column, by the time you get to the right column, you should have a 23 or so column. It's like this point was made for this achievement. If you miss it, it's only a minute or two into the level.