Centurion CN9-A is a Medium-class 'Mech in Battletech.
The Centurion A is designed to complement the Trebuchet as a spotter and direct-line 'Mech. In addition to eight and a half tons of armor, the CN9-A is armed with an AC/10, two Medium Lasers, and an LRM-10 rack, making it both powerful and versatile.
The Centurion sets the Gold Standard for a generalist "Trooper" mech, with a flexible, all-purpose weapon load and hardpoints, good payload, decent armor, and acceptable but not outstanding speed. Its payload and hardpoint count make it applicable to truly any role your particular lance needs, as long as its speed isn't a hindrance to that role. What you choose to use it for will depend on the rest of your lance:
If the Centurion is currently your biggest 'mech (such as in Career mode), it makes an excellent spearhead: bulk up armor to max or near-max and pack the Missile hardpoints with SRMs with Medium Lasers as backup. Jumpjets are useful for getting the slow-ish Centurion into close range to use these weapons. The biggest decision to make for a setup like this is juggling whether to add an Autocannon, to increase range and single-location punch; or spend the weight on heatsinks to keep the close-range battery firing.
If your lance is already well-balanced with your other 'mechs, keeping the CN9 as a generalist suits it just fine. In this case, it comes down to preference what exactly you want out of your Centurion between range, firepower, armor and jumpjets. An AC/5 for the Ballistic mount is a good starting point; after that, what you do with the Missile hardpoints will be most important. Its 3 hardpoints and high payload mean the CN9 can carry an acceptable battery of both LRMs and SRMs at the same time if desired, with the split being your choice. You may also choose to skip the Autocannon entirely to bring larger missile racks. If your weapons are split between different ranges, heatsinks are less necessary since you likely won't be firing the entire weapon load at once, since at least some of the weapons will be out of their ideal ranges (this is known as "bracket-firing" and a 'mech built this way is often called a "bracket build").
If the Centurion is one of your smaller 'mechs or you're often up against 'mechs larger than it, its slow speed makes it less suitable to hit-and-run skirmishing, even with jumpjets. Fortunately, its high payload and favorable hardpoints make it well-suited to a long-range fire support 'mech with a long-range Autocannon and LRMs. A build like this tends to run cool as well, allowing you to use the weight saved on heatsinks to add jumpjets, armor, or more weapons.
The stock loadout leans into firepower with the AC/10 at a cost to armor; this is a frightening weapon to anything smaller than the CN9 and means even heavies have to respect it, but early on your pilots will likely have poor gunnery and recoil control, and misses with it are painful considering the tonnage spent on it. Later on, once you're starting to face heavier mechs, you'll have a decision to make: keep the heavy cannon and shift the CN9 into an ambusher (because its merely decent stock armor will become a liability against opponents with heavy-class firepower); switch the cannon to a lighter autocannon to bulk up the CN9's armor; or switch to a longer-range setup with a long-range cannon backed by LRMs to keep it away from danger.
A quick optimization on the stock loadout is to switch the LRM-10 to a pair of LRM-5's and a heatsink. Two LRM-5's are a ton lighter than a single -10 but firing them both generates 2 more heat; however the extra heatsink adds 3 sinking that will either 'profit' heat or benefit the other weapons.
This is a simple tweak applicable to many 'mechs that have a single LRM-10 on their stock loadout but have a Missile hardpoint to spare.
Dump the LRM-10 for a pair of SRM-4's. If you want to skimp a bit on the armor, make it a pair of SRM-6's. It could well be your very first brawler. AC/10 + 2 SRM's is some serious stability damage, as well as some real pain, especially in the early to mid game. That AC/10 is going to punch through the armor of whatever it hits, and the SRM's have good odds of critting, at least until you start running into Heavies. It is a mech to grow out of, but a solid and dependable one.
The Centurion can be "upgraded" into a serious close range brawler with some cheap parts. Downgrade to a AC/5 with one ton of ammo with a small laser underneath, replace the LRM with two SRM/6 and one SRM/4 for a total of 16 SRM tubes with two tons of ammo, keep the two medium lasers and a single heatsink. This makes a Centurion that can do a stupid amount of damage at close range for about two turns (which is when it over heats) and also has more armour than the stock model.
A single AC-20 with two tons of ammo will make a good light mech hunter-killer. Few light mechs can take two AC-20 rounds and live, and even fewer can survive a direct shot into the center torso. There is also enough weight left over to mount the maximum amount of armor, along with jump jets, allowing for additional mobility and tankiness.
Alternatively, the LRM can be removed for jump jets and additional armor, making the Centurion a mobile generalist.
Going in the opposite direction, the Centurion's 3 Missile hardpoints and high payload give it the highest potential LRM barrage of any Medium (barring the LosTech Griffin). It can serve as a very effective early-midgame missile boat until you find a Catapult or Jagermech-A to provide a heavier barrage. The Kintaro is another option, paying 3.5 tons of payload for an increase in speed if you find that desireable in your LRM-boat.
The Centurion is a good allround medium 'Mech for the early game. It has good tonnage and armor, allowing for multiple loadouts to be fitted to it. It's a bit more fragile than later medium 'Mechs so it will be discarded when you can reliably get Griffins; though with its higher available tonnage and lower speed than the 55-tonners, it still serves well as a longer-range fire support platform with heavier weapons.
A sturdy and reliable 'Mech, the Centurion is good at both taking and giving a beating. CN9-A models are able to engage at any range, but their damage curve increases as they approach a target.