They're big, tough, and nasty, but don't get suckered into the mentality that they're supposed to be invincible game-enders. The people who expect that are the ones who are disappointed by them, and are the ones you'll see on the forums declaring their uselessness at every turn.
Unfortunately, that false reputation works against them. They're not built to absorb multiple lances' worth of fire, but because of their reputation, that's exactly what people are going to point at them. Consequently, you have to fight smart with them. Cover is going to be hard to find for something that tall, but that's exactly what you need. Keep maneuvering so that he can't get all of his guys on you at once. Those little missile racks aren't really going to contribute that much to your damage output, so I like to load smoke rounds in either the SRMs or LRMs and use them to help make my own cover. I know, smoke rounds only make a 2-level-tall column of smoke, but if you put that smoke on top of the Level 1 hill that previously wasn't doing you any good, it'll give you some much-needed to hit modifiers, if not help block LOS completely. Also, consider going hull down or even prone on a regular basis. That'll help you take cover behind normal-sized terrain. Finally, don't forget that your sheer size gives you some movement bonuses, so feel free to plow your way through woods or other terrain that will slow down your opponents, hopefully giving you more opportunity to split them up and defeat them in detail.
Similarly, remember that like the armor, the Ares's firepower is nasty, but NOT apocalyptic. Unless someone is dumb enough to get into melee range, don't expect an Ares to strike down an enemy 'mech every turn with the wrath of an angry god. What you do have is a solid and consistent damage curve. You're not going to one-shot much, but anything you do go after WILL know it was hit. Since we already discussed splitting up enemies, make sure to put a solid salvo into anything that can see you. Between your fire and that of your buddies(if you're not giving your tripods a very healthy escort, you deserve the defeat you're about to receive), you should quickly find yourself facing not a horde of enemy Battlemechs, but a gaggle of the walking mostly-dead. Again, take a look at your missile racks, and see what kind of force-multiplier ammo you can carry that'll help them transition form mostly dead to ALL dead. Finally, take advantage of the fact that you're probably not fighting alone, and that most of the enemy's attention will be focused on your Ares. While they're coming to grips with your Colossals, what are your other guys up to? More importantly, whose back armor are they looking at, since nobody's paying attention to them?
