When you unleash a series of attacks with a weapon by pressing
repeatedly for your primary weapon or
repeatedly for a secondary weapon, you execute several regular attacks culminated by a finishing move. This is an attack chain. Attacking enemies with 100 of such attack chains with any weapon unlocks this achievement. The finishing move in the attack chain must connect for the chain to count, but the earlier attacks need not connect.
Players, most likely, experience their first attack chain in the prologue when they are introduced to the longsword. Since the chain is achieved by spamming the primary weapon attack button or the secondary weapon attack button, attack chains will be performed in the normal course of play. Consequently, this achievement should unlock fairly early in the game.
The chakrams are an easy weapon to use for this achievement because they are a midrange weapon. It is simpler to complete the chain without being interrupted as might be the case with a close quarters melee weapon. In the very short video below, The Twinned Souls chakrams are used as the primary weapon. It is available at the beginning of the game when the player enters the village of Gorhart and opens the special delivery chest. Two attack chains are shown in the video. The second is the character's 100th chain and unlocks the achievement.
Note that when the player levels up and acquires special weapons abilities, these also count as attack chains. However, the achievement would probably already have popped before these special weapons abilities are acquired.