Let's assume, like bombardier beetle your creatures can produce instant violent chemical reactions inside their body and launchs hot mix of acids that strong enough to eat away and penetrate the armour or render the tank incapable to operate.
Or
Instead of launching hot mix of acids, It launchs biogical armor penetrating projectile.
Or
Since most of the modern high explosives are organic compounds (I'm confident enough to say one determined genetic engineer can manipulate an e. coli genome to secrete Trinitrotoluen -TNT- ).
Like dragons breath fire let's assume your creature can manage to produce enough "explosive dung" and launch it with one fart to turn the tank upside down when exploded underneath it.