TCoM: Archolos is a total conversion of Gothic 2 so it uses the same controls. Press and hold LMB + press whatever you've bound movement (Forward, Strafe left, Strafe right, Backward) to.
Pressing and holding LMB + Forward does a standard slashing attack (pressing Forward again at the right time does a follow-up thrusting attack; higher weapon proficiencies unlock a third and fourth attack).
Pressing and holding LMB + Strafe left/Strafe right does a standard swiping attack from either left to right or right to left.
Pressing and holding LMB + Backward allows you to block incoming attacks (not sure but may be timing-sensitive as well).

All of those can be chained together in any order. With no/low weapon proficiency executing and chaining together the attacks will be rather slow and feel unresponsive (mirroring the character's lack in experience with fighting and handling weapons) but gets progressively faster/more responsive as you gain more weapon proficiency. Press and hold LMB + press Forward.
At no/low proficiency in ranged weapons you'll miss the target quite a lot though. Higher proficiency raises the hit chance, the critical hit chance as well as extends the effective range of targeting. Raising Dexterity also increases range + damage.

Gothic 1 and 2 are meant to be played with keyboard alone. Use action button ( Ctrl ) + keyboard arrows to attack and block, as well as jump button ( Alt ) + keyboard arrows to strafe.

There is a minor difference between games. In Gothic 1 and mods, you should keep action or jump button pressed until intended action is performed. While in Gothic 2 and mods ( including Archolos ) you should not ( otherwise, the same action will be constantly repeated ).