This trophy is quite simple and can be unlock purely by accident given the amount of fun the ability is. however if people are struggling to get this i have some tips that could help you to get it;
1. the trophy description explains what it wants however what it doesn't explain is that you can use the other abilities but they can not take down any enemies otherwise the counter resets.
2. the game tracks your consecutive take-downs in the challenge area; so if you have no idea how many in a row you have dealt with you can always check there
3. while it may seem like you can't do it at first due to lack of ammo the game doesn't tell you that you can refill your heavy ammo at any neon sign; the size of the sign affects how much you get out of it; so a small one only offers 1 heavy ammo and so forth.
4. you don't need to do it in one shot; it doesn't matter if there isn't 20 enemies lining up for you to take them down; so long as you take-down of them in a row they can be spaced out pretty far from each other.
all in all i hope these tips can help someone who might be struggling with this trophy