First of all, I'm quite sure that SETENTITYIMMUNE and DAMAGEBY are not vanilla FPSC conditions, they either are from another engine or an FPSC mod.
However, there is a way your script might work.
1. Give the tank (which has to be a dynamic entity) 100.000 health.
2. Give the panzerschreck 1500 damage in the gunspecs
And then use this script:
;Artificial Intelligence Script
;Header
desc = Tank Destroy only by panzerschreck
;Triggers
:state=0:state=1
:state=1,shotdamage=999:state=2
:state=3:explode,destroy
;End of Script
The SHOTDAMAGE=X condition is fulfilled, when the damage of a single shot at the entity exceeds X. So with conventional weapons that usually do 10-50 damage per shot, the player will most likely run out of ammo before the tank eats up 100k damage while a single hit (even a strafing one) by the panzerschreck will trigger the destruction.
In case you find my grammar and spelling weird ---> native German speaker ^^