Here are some Armored Core mech names fitting the description:

Emphasizing Reverse Joint Leg Style & Mobility:

• Gyre-Stalker: Suggests swift, circling movements.
• Contortionist: Highlights the unusual leg articulation.
• Backlash: Implies powerful recoil from the reverse-jointed legs.
• Recoil Serpent: Combines the leg style with a sense of fluidity and danger.
• Inverse Momentum: Focuses on the unique kinetic energy of the design.
• Aetherial Weaver: Evokes a sense of graceful, unpredictable movement.
• Paradoxical Sprint: Contrasts the unusual leg design with incredible speed.

Emphasizing Heavy Weight & High Mobility (a paradoxical combination):

• Iron Colossus: Highlights the weight, but hints at surprising agility.
• Juggernaut-X: The "X" suggests a modified, more mobile version of a heavy mech.
• Momentum Breaker: Suggests the mech's ability to overcome its weight.
• Gravitational Defiance: Emphasizes the unexpected mobility for its size.
• Inertia Crusher: Combines the weight with the ability to overwhelm opponents.

Emphasizing Stagger Strike Combat Style:

• Seismic Hammer: Suggests powerful, earth-shaking blows.
• Ground Shaker: Similar to Seismic Hammer, but more concise.
• Stagger King: Directly references the combat style.
• Aftershock: Implies the lingering effect of the mech's attacks.
• Concussion Cannon: Focuses on the impact of the stagger strikes.
• Shatterstrike: A more aggressive and impactful name.

Combining Elements:

• Gyre-Juggernaut: Combines the circling movement with the heavy weight.
• Contortionist Colossus: Combines the unusual leg style with the immense size.
• Recoil Serpent-X: Adds a modification element to the fluid, powerful design.
• Inverse Momentum Breaker: Combines the unique movement with the ability to overcome inertia.
• Seismic Weaver: Combines the powerful strikes with the agile movement.
• Paradoxical Shatterstrike: Highlights the unexpected combination of mobility and powerful attacks.

These names aim for a balance between evocative imagery and a sense of the mech's capabilities. The best choice will depend on the specific tone and style of your Armored Core project.