Well, then it's not a question of "you MUST kill the field guns" but rather "I kill the field gun because it's more expedient".
That is, of course, quite a difference.
That said, reducing fortification with a tank gun sounds like pain indeed. But a gal with a vibroaxe can free such a tank in half a turn.
MUST kill, because if tank will stuck a few turns, other tank might take too much damage, too much damage will take also other part of your team AND those idiot NPCs, absolutely insane moving under bullet storm. As result - more damage to team (and it's almost global defeat on highest difficulty) and less rating (which is the same).
Really pain.
A gal with it CAN be solution, but again, there is couple moments for it:
1. You must have that axe and gal, who can effectively swing it. I have no melee gals, only syn and one lokk'nar
2. Even in case you have a gal with axe, it must spawn near the tank. So... more pain.
The only and obviously easier way - save game right before landing.