Combined Naval Air Tactics

It's better to use strat bombers to hit province buildings and tactical bombers to attack ships or army units, Interceptors are more for attacking enemy aircraft, hence the name, they aren't built to attack ships or buildings.

A max level destroyer is meant to assist the bigger ships that are built to take on cities and other ships. Destroyers are meant to protect them from submarines and aircraft.

Interceptors are used to attack other aircraft not ships or buildings.

But I currently only have intercepters and destroyers available, and I'm facing substantial infantry buildup at my border (stack = 13). At the same time, two enemy destroyers are roaming the sea along the coast. I already ordered artillery bombardment on the infantry, but that only creates a small dent. I don't know what to throw at the stack to destroy it quickly..

Destroyers don't have any range, they will have to get close to bombard your cities, you could also wait it out and build up a proper army, if all you have is interceptors and destroyers then you haven't been doing the right job. More often than not the first thing you do is build up land defences and then think about force projection.

Oh, they are all not I have. Got some groups of regular infantry, supported by armored cars. Currently building light tanks in 3 different places. But the game's tutorial and mechanics are not always very clear (and its my first game), and I was just looking for the most effective way of destroying a heap of enemy infantry in plains. Bombers are being researched, but will cost a day or two before I got one built.

EDIT: so destroyers CAN bombard an enemy infantry stack? Will the enemy infantry be able to fight back?

Destroyers can bombard things that are on the edge of land their range isn't anything to be proud of, What I suggest you do is engage the destroyers with either the tac bombers or your destroyers and use the interceptors to survey the enemies movements, be defensive until your ready to attack.

Destroyers can not harm anything on land except aircraft. They can bombard it but it is like scratching a tank with a knife(it does nothing). To quickly destroy them, I would advise either using a cruiser or a battleship with destroyers for protection from subs and planes. As for infantry, your inf and cars should be able to handle it as it is easier to defend than attack.

Then i would have to research them first (its only day . Guess its faster to send in bombers then.

Yep. Good luck.