Turtle Breaker Tactics

The Roman Legionaire was weighed down by plate armor and a large shield. He was, in effect, medium to heavy infantry. Unless mounted he could not move very fast, which is partly why the Romans built the large imperial road networks.
While advancing upon, say, an enemy army equipped with archers, the Romans employed the testudo formation, which was basically invincible in its day.
So I was thinking, what would've been a way to break the testudo? Here's an idea I had.

Cavalry formations charge the testudo. If the Romans break formation to engage normally, the cavalry can fall back. If the Romans break formation to throw their javelins at the cavalry, the cavalry can either outrun the heavy projectiles or engage with their own javelins, which by design could not be used again once thrown If the testudo holds, the cavalry proceeds to throw grappling hooks on top of the shields, and then it falls back. The weight and power of the horse is enough to force the shield out of the Legionaire's hands and drag it away, and after that the Legion is vulnerable to javelins, which are heavy enough to pierce Roman armor, especially combined with the fact that the thrower's on horseback, which I would assume add velocity.
Thoughts?

Dassault Papillon: The Romans faced armies who tried exactly that tactic numerous times. Such armies generally forgot that pilums are especially deadly against cavalry, either as spears or javelins.

Of course some armies DID fall back in response to this and even learnt how to shoot arrows while doing so. Such armies were pretty effective IF the Roman army could be caught without their supplies. Otherwise the testudo worked fine at blocking most arrows.

admin: The grappling hook trick, or just the "trick them into breaking formation and then attack" tactic?

Dassault Papillon: Kinda assumes the roman legions would break ranks, but you're basically describing Surena's strategy at Carrhae.