You'll spend most of your time running and gunning in The Ascent, though there are other options that can be unlocked as you play. Specifically, you'll want a melee attack to deal with rushing enemies, or to give you options while you reload. You won't start with a melee attack in The Ascent, but through Augmentations you can at least unlock them. Here's how.
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The Ascent: How to Get a Melee Attack
To get a melee attack in The Ascent you will need to unlock the Hydraulic Slam Augmentation. This is usually picked up during the first couple of missions, but if you don't receive it you can head to a Grafter. These vendors can be found on the map indicated by a blue DNA icon. just head to them and select 'Browse Chrome'. From here you'll be able to browse Augmentations and purchase them. Before leaving, make sure to go in for surgery to have Hydraulic Slam installed.
How to Perform a Melee Attack
Once you have the Hydraulic Slam Augmentation equipped, just press LB or RB to trigger it. It'll knockback and sometimes flat out vaporise enemies in close range of the attack. Use it to push back large groups and rushing enemies. Doing so will deplete Energy though, so keep an eye on the blue meter next to your health.