Firstly, the A/C-20 has 18 hit points, most of anything in the game currently, but since it takes up the most slots, it also gets hit the most, and that makes it more fragile. With so little armor, opponents will usually go for that shoulder anyway, which is why it dies often.
Oh that mech is not very good. It needs a lot of stuff. First I will improve it, and then I will give advice on a 'better' (what I would do) way to run it.
I've made something like what you have, in terms of numbers (surely you have the armor all spread out). There is 8 tons left over, which I suspect is for heatsinks, and ammo, for now, I will fill it with what I would consider the best, if I had to use the same mech.
So to start, I have this:
http://mwo.smurfy-ne...1c232d6ebc8086c
There are a few things you HAVE to do:
Get Double Heatsinks, these are almost always a pure upgrade and will make heat management much easier.
Get Endo-Steel, you have (I suspect) 24 free slots, which are wasted. With Endo, you could get more tons and be more dangerous.
Get more armor, in fact, almost max. Don't run anything short of maximum (minus a few points to give you a half ton or full ton), at least while you start off.
IF you have 14 slots left, which is unlikely, get Ferro Fiberous armor as well, its like a worse version of Endo Steel.
Here is a straight upgrade it could even use FF armor, which we will ignore for now, and increase its engine size. It loses 1 ton of ammo currently for its A/C-20. If you ignore my other opinions, at least do this. Remember, ammo placement and armor placement is your choice, I just jam them in places. I will give you a full loadout when I'm finished here.
http://mwo.smurfy-ne...1c232d6ebc8086c
Now onto weapons.
The A/C-20 is a great weapon, but usually requires closer range, for this I suggest:
Better close range weapons
Higher engine if possible.
A/C-20s usually only require 3 tons of ammo, and a 4th is great. 5 is probably too much.
Your ER PPC is good for sniping, but is too hot to really be effective at close range, and your ER large is too hot as well, and not as good at longer range as PPCs. If you really want to do the Sniper+A/C-20 thing, I would suggest removing them and putting 2 PPCs instead of ER PPC and ER Large, however, most mechs are best when they go to their strengths, for this reason, I suggest either 3 Large Lasers, or 2 Large Pulse Lasers. I'm found of LPL, so I will be using them. To make room, I will remove the AMS, because AMS is not only terrible, in the current state of MWO, LRMs and other missiles are pretty terrible, so its even worse. Most mechs should avoid AMS, at least currently. Because of the simplisity of the mech, I added FF armor as well.
http://mwo.smurfy-ne...e8ba60e93cef36c
Although that mech is pretty powerful at close range, it has some extra tonnage still, and that .39 tons, I will be taking armor off the legs (usually the best place to strip if you are not a light mech) to make it a full ton. This is also were I will place the ammo, as people avoid shooting legs off bigger mechs. Make sure you don't strip too much, as people will notice and you will die to it.
http://mwo.smurfy-ne...e5d9277396642d7
This is the mech I would run. I would consider leaving 3 tons of ammo, and no HS in the engine, and increasing engine size, but in the interest of keeping it overall cheaper, I left the 280 you started with.
Hope that helped