Ok, there are a number of things that you can do in a situation like this.
First, and the most cumbersome/least effective - assemble a giant stack and attack. Attack with summons first, then mop up with the rest. As it appears that you already attempted this, this doesn't appear to work.
Second, get the cover promotion to wipe out the archers. Cover requires a melee, archer, or mounted unit to have combat 1 and then take that as their next promotion. Disciple units may be able to take it too (not sure on that one). You will probably still need a number of units to do it, but it gets much easier.
Third, ranged attack. This is fall further, so your archer units have a ranged attack. Spam archers and then use the ranged attack to whittle away at them.
Fourth, collateral. Collateral can come from two things around this stage of the game. First, it can come from catapults, available at construction. Since this is FF however, catapults are less useful (they can still be used, but aren't as useful) outside of attacking cities. Second, it can come from magic. Magic that can be used to cause it are maelstrom (air 2 spell), fireball (fire 2 spell), ring of fire (ritualist spell), and a few unit specific spells. To get these spells, you need a unit with the channeling 2 promotion. As you appear to be playing as d'tesh, that means that you need to get a unit to level 5 (17xp). With other leaders you only need to get to level 4 to upgrade.
The best way to take out a stack like that is to hit it with as much collateral causing damage as you can, pick away at it with ranged attack to further weaken them, then hit them with your troops. Generally, if your best troops have less than an 80% chance of victory, I'll hit my opponent with my worst troops first, working my way up till I can kill them with my better troops.
-Colin