FB White in the bottom room and FB Glacier in the upper central room, easy peasy lemon squeezy

Play solo against Cerberus, using a kit that can take a few hits, Krogan Battlemaster is who I normally use but played well, any kit could be OK.
Play Gold if you can handle it, silver if not, hop over the counter when they come looking for you and grab them before they move away.
If you're lucky several units all line up to be grabbed in the same place, my longest streak in a single spree is 4+ on the trot.

When other units come around to meet you on the other side of the counter, you simply hop back over and repeat the process.
Extra Info:Assuming you're trying to complete Combat Mastery, there's no need to try and complete grabs by doing a solo etc. 1M melee points is a lot and in both my experiences I always maxed out grabs before reaching even 500K.
You only need to tap the grab button, it's not neccessary to hold it down, in fact that usually messes things up