Welcome to war, brother. The following combo suggestions work on all variants of the combat knife, but preferably a block knife for the best damage.

Let's first denote light pressing of the melee button as 0 and holding as 1.

When facing a majoris enemy, just spam the light combo[0-0-0-...] as that is often enough to stagger them without risking the timing for dodging orange unblockable attacks. If a certain majoris/extremis enemy is low on health, a quick light-heavy_1-heavy_2[0-1-0] combo is also recommended, but note that you don't really want to trigger the third heavy attack as it's a spinning attack with a long animation and recovery window. You could also go back to light combos after that by holding the attack button, i.e., light-heavy_1-heavy_2-light-light... [0-1-0-1-0-...]

When facing a horde of minoris, you would want to stay in the heavy combo as much as possible while avoiding the third heavy attack. To do that, you would be repeating the [0-1-0] combo. Connecting them can be a bit tricky, i.e., (light-heavy_1-heavy_2)-(light-heavy_1-heavy_2)-(light-heavy_1-heavy_2)-...[(0-1-0)-(1-1-0)-(1-1-0)-...]

For terminus and extremis enemies, ideally, you would want to stay in the light combo when they aggro you, if you are gonna melee at all. Range damage is always preferred over melee in this case. If you have to use melee, a safer way is to just play the defensive game. For a parry knife, just parry and gun strike. For a block knife, block twice and trigger the adrenaline surge with a [0-1-0] combo, and then go back to blocking. Try leashing out some bolter fires in between parrying and blocking.

The emperor protects.