Well you either give them a snare or you buff Piats to not be so inaccurate. This issue is a relic design issue.
Whats the damage of the PIAT vs SHRK vs Zook?
They do 120 dmg, just as much as a Schreck. A zook does 80 damage.
The problem however is not the damage, it's the fact that PIATs do deflection damage of 80. That means your PIATs don't need to penetrate in order to do damage.
That means all you need to do in order to take out heavy tanks with high Armour is to snare them and blob enough PIATs, the tank will eventually get destroyed after a few seconds, since a PIAT shot will always do damage. And Piats are cheap as fuck with 40 ammo cost.