A montage is a collection of the best clips that you have to offer. Usually it is set to music, and edited with transition effects or something similar. A montage is something that can take up to 6 months to collect good clips. For an average player, a montage clip is a clip that is your best ever, a clip so good that you brag to your friends about.
A montage is not something that can be whipped up in a couple of days after the game comes out, it can take months to get a legit montage.
For this game, a montage should have things like triple and quad feeds, crazy kill (like grappling hooks) and other things that you don't see every day, not average gameplay set to music.