Casse aren't horrible...I crushed a phalanx army with the Casse very recently. There are a few things I discovered:

1. You have no heavy cavalry, hence chariots are incredibly important to defeating heavy horsemen. You have to protect them and divert enemy attention away from the chariots, so you can spring them at an opportune moment and crush your enemy's cataphracts, or whatever they have.
2. Keep your opponent out of his comfort zone. That means using unconventional tactics, particularly attacking at an angle, and avoiding linear engagements. Your troops are lighter and not of the same quality, so you will lose if you go at the enemy in a conventional linear fashion.
3. Use morale to maximum effect. The Casse have several units that offer the "command" bonus to local troops' morale. This is crucial to keeping your men fighting, as Celtic units don't have the same morale level as Germanic and Getic ones - but if you can keep your men fighting, your units have greater numbers and high-lethality swords to grind your opponent out with. At the same time, take note that the Ebherni Shock Troops now frighten nearby enemy infantry.

Here's the build I brought in that recent battle, to the best of my memory, with an analysis accompanying it:

1 Chariots - Explained earlier. Also note that they have the inspire nearby troops "command" ability.
3 Goidic Cavalry -I chose these because they have a combination of light horses (letting them outrun almost any enemies) and acceptable staying power (for barbarian javcav, that is). They are relatively expensive - 2700 mnai - but it was not too big of a deal as the rest of my army was fairly cheap.
4 Immanae - Two purposes: javelin harassment (particularly on the flanks), and meatshielding. You have to save your swordsmen, chariots, and cavalry from suffering arrow barrages, hence these hapless hooligans are sent to take the hit.
1 Sotaroas - Same purpose as the Immanae. With numbers expanded to 80 men on large, they do this job even better now, for not much more cost, and can get a couple shots off at valuable, easier to kill targets like medium cav.
1 Kluddargos - Have the command ability, plus wield brutal swords. Can be vulnerable, as they lack shields, so be sure to keep them in melee to make hitting them difficult.
2 Dubosaverlacica - Lower nearby enemy morale, have armour piercing javelins, and can take on almost any single unit and live to tell the tale. Be sure to not waste them - but don't consider using them as a flanking force a waste. Having two of them helps, so you can commit one and have the other sitting right behind the main fight, still giving off the fear effect, but ready to go at any time.
8 Botroas - Yes, 8 of them. They're extremely cost efficient, as they have 100 men, three javelins, and 0.225 lethality longswords. A perfect unit for massing in a critical attack on an enemy.

Finally, one more tip: be patient while setting up your attack, but once you commit, be aggressive and strike hard with all appropriate resources. Here's some reasoning behind that:

-You need your attack to work, as you will have to be the attacker, since if you're passive, the enemy can easily set up his 'superior' forces to overwhelm you. Hence, you must make sure you are ready to commit when you do so.
-Having positional superiority is crucial. The more enemy units you can effectively ignore, the better. That's why an angled attack on one flank works - much of the enemy's force simply can't be brought to bear without messing up his position. Keeping a few units in position to either reinforce your flank attack, deter enemy movements, or troops from one flank coming to another's aid - is a good way to take advantage of this.
-Committing all appropriate resources to the attack means sending everything to ensure victory at a crucial point in the battle. Don't feel this is overcommitting - you probably don't need to send your whole force to ensure this, and if you feel it's still too much, remember that if you don't win here, you're going to lose anyway. This means sending three cavalry units to deal with his one. This means using three units to kill one phalanx. Local superiority is the crux of a successful attack in all warfare; EB Online is no exception.