There are two ways you can do this one. You can actually play through the combat simulator on Normal/Hardcore/Insanity and get 9999 points. Or do it the easier way as this guide will describe.
This guide assumes you have all the Arena settings from the Armax store.
I turned my difficulty to Narrative; though I'm sure casual will do just fine for others.
Turn on all of the score modifiers.
Choose a map (Wingman seems to be the easiest to navigate).
Choose enemy: Reapers, then the option of super elite.
If you have the M-7 Lancer, you are going to want to equip that because if you didn't know, it has infinite ammo. Feel free to keep the player damage boost score modifier on; though it will deduct from your overall score.
Squad members have infinite ammo, so you don't have to worry about them.
I was an Engineer using Garrus and Miranda. Honestly, the squad members don't matter much, I didn't see them use powers at all, so I wouldn't rely on it unless you are someone who activates powers for squad members. Sentry turrets and combat drones are your friends.
I don't have any special strategy for hiding or anything. Play the simulator like you would normally. At the end, when you are around your last two enemies, change your combat level up to normal. Kill those enemies (and be careful that they don't kill you instead) and watch the trophy pop after you load back into the lobby.
Bonus's can help; I got them. Though I didn't get a kill streak higher than three. Doing all of that, my score ended up being around 12k.