Weak Point Gauge will appear over the head of an enemy or boss after exploiting its weakness or after immediately counterattacking. Landing a hit while this gauge is up will stun the enemy, allowing you to land more hits. If you're able to break the gauge, your character will launch a special, powerful attack that will deal heavy damage to the enemy. This feature is pretty useful against bosses since their innate high defense will prevent you from inflicting that much damage with your normal attacks.
When "breaking" the enemy's weak point gauge, it's recommended to stick with basic combos. The rate on how easily an enemy's W.P. gauge breaks varies by enemy. Some enemies get their gauge broken with just 2-3 hits, while others, like bosses need several combos to break.
There are two ways of getting the enemy's weak point gauge to appear:
Different strategies apply to different enemies and bosses. Keep your fingers to the evade or block button whenever you're trying to observe an enemy's attack pattern. Usually, bigger and slower bosses need Tools to exploit their weaknesses while smaller and faster bosses (like Volga) will require you to counterattack right after they finish a combo or signature strike.