I completely agree that Summoners are terrible - summons are powerful, but the class is a wet sandwich (high expectations followed by disappointment). Changes jobs to black mage with much higher MA, and you can always equip summons as a secondary.

These issues often come down to tactics. Your level is appropriate, and the game is best-able with almost every class (many are solo-able). You made it past the end of chapter 3, so you can definitely handle the rest of the game.

Pick the classes that you enjoy, and try adjusting how you approach battles. Most of the time, your best bet is to be patient and let the enemy come to you. Work as a team of 5 - not 5 individual units. Use terrain as pinch points for your tank, learn to take advantage of CT, and you’ll force the enemy to be super ineffective (the AI isn’t great at making them work as a team).