Sandcasters!
These special missiles are full of little lead pellets. They're They're actually intended for intercepting missiles, but the then-pilot "Sandy" Gray realized that there's nothing stopping you from launching them at the enemy.
A one gram projectile moving at .999c carries a ton of energy; about 500 kilotons. You You can probably pack a million pellets in a missile. With With a million little chances to breach the shields, one is bound to get through. As As a bonus, it's basically impossible to dodge.
DISCLAIMER: I'm not a physicist and this might be wrong.
$E_K = \left( \frac{1}{ \sqrt{1 - v^2/c^2 }}-1\right) \times m \times c^2$
$E_K = \left( \frac{1}{ \sqrt{1 - 0.999c^2/c^2 }}-1\right) \times 1g \times c^2$
$E_K = 1.887×10^{15}J$
Wolfram|AlphaWolfram // NUKEMAP
Godspeed, Admiral GraySandy.