Make sure that you are:
1) Taking advantage of the terrain. Use chokepoints to limit the amount of enemy soldiers who attack at once and protect your marksdwarves. Ensure that enemy marksmen don't have much time to loose volleys at melee. Sure, twenty enemy squads seems like a lot, but if you make them take a long way to reach you, often you can rout some groups before the rest get there, while increasing the amount of time your marksdwarves have to empty their quivers at them.
2) Effectively controlling your troops: letting a few soldiers charge out at once is a recipe for death. Never let your troops scatter and get surrounded if you can help it. Isolated troops are probably dead.
3) Training and concentrating elite troops: when the enemy are coming in such numbers, you cannot rely on quantity to defeat them. You need to concentrate your best warriors and tear apart the enemy squads as they reach your army, so that the rest of the enemy force loses morale, breaks, and flees
4) Using quality weapons, arming and armoring all your warriors, and using superior metals. Shield and weapon bonuses alone can dramatically increase survivability. The Fortress Defense Mod is designed to force you to use all the tools available at your disposal.
That said, don't forget that removing civs from Fortress Defense is an option if you think some of them are too hard; the Fortress Defense Mod only exists to provide you with what you consider Fun after all
Unfortunately, guaranteeing war between civs is impossible; civs can be made to hate each other, but cannot be forced to go to war short of the workaround that Fortress Defense uses. The best that can be done is to make the ethics of two civs oppose each other diametrically, which is still far from certain to work, and a little complex when you're working with the number of civs in the Fortress Defense Mod.