Easy wins:
Target your opponent’s legs. Although they are harder to hit, they are less armored than the main body. Also, if you can shoot the legs out from under a Mech, odds are you will be able to salvage the chassis at the end of the mission.
Avoid overheating:
Be aware of the environment when you outfit your Mechs. Avoid weapons that generate too much heat when you will be battling in the desert, or else you will have to shut down after every other shot to cool down.
Keep your heat efficiency in the green zone, but just barely. If you know how to manage alpha strike fire versus single fire, you can lose more heat than you make, ensuring a nearly constant fire rate. Too many heat sinks will waste space.
Weapons:
Pulse lasers are nice, but can overheat your Mech if you are not careful. Balance pulse lasers and single shot weapons.
The PPCs, Flamers, and LBXAC20s are recommended. Although a good variety of weapons is necessary for any Mech, these three weapons are particularly versatile. The PPC fries electronics and does amazing damage. It is a great assault weapon and a critical one for many missions. The Flamer does a nice amount of short range damage, but also creates a lot of waste heat in the target Mech, forcing some opponents to shut down. And the AC20 is the king of damage for in close fighting. If your Mech can mount one or two of these weapons, do so.
Jump Jets:
Forget Jump Jets unless you are skilled at using them. Light Mechs move too fast and stay in the air too long, and heavy Mechs cannot stay up very long. If you get rid of Jump Jets, you will free up a good amount of weight.
Chassis:
If you must have any Chassis components, go with the AMS or LAMS systems. These will help foil missile hits, and when fighting heavier Mechs, this is invaluable. The Beagle probe only helps missile-heavy Mechs; only use it for Catapults and the like.
Armor:
Armor only as much as necessary. If your particular weapons loadout can be stuffed into just the torsos or the arms, you will not need armor in the empty areas. They do not particularly affect your vital areas, and you will not lose any weapons. Also, you will free up space for more ammunition or heat sinks.
Recommended Mechs:
For an easy Instant Action win, build a Mech with double Thunderbolts. In battle, run up to your enemy and fire them at his cockpit. This usually works in one or two hits.
The following is a good long range Mech. Get a Mad Cat Mk II and strip it completely. Lose the Jump Jets, Heatsinks, weapons, all the armor, tone down the speed to minimum, etc. Add two Clan Gauss Rifles and two Light Gauss Rifles, raise the speed up a notch. and put the rest in armor. Use the following firing chains. Note: Keep this Mech away from close combat and always try to hit the cockpit. If you manage to knock a mech over, blast it with all four at once on the cockpit and watch it burn.
1: One Clan Gauss Rifle and one Light Gauss Rifle
2: The other Clan Gauss Rifle and other Light Gauss Rifle
3: All of them
The following is a good short range Mech. Get a Daishi and strip it down so it has no armor, no speed, no extras, etc. Put a Clan LBX AC10 in each arm, a Clan LBX AC20 in each torso, and a Thunderbolt in the center torso. Top out the armor and add as much speed as you can.
The following is a recommended Clan Mech. Get a Shadowcat and strip it down to minimum weight. Next, put two Clan LRM10s, two Clan RM5s, two ER large lasers, set the speed to 87 km/h, Jet Pack, Beam Activation, and LAMs. Whatever remains can go to armor, heat sinks, or speed. When it is done, the mech can destroy buildings with usually one hit, and enemy mechs with a few hits.
The following is a recommended beam weapon Mech. Start with a Nova Cat mech. Strip it to the minimum, and put in two Clan Large pulse lasers, four Clan medium lasers, and fill the rest with Clan Small lasers. Put in 5 tons of heat sinks, and fill the armor slots. Do not worry about speed, as no matter how fast you go, you will always have at least something firing.
The following is a recommended long-range Mech. Take a Daishi, strip it, and put a Clan Gauss in each side of the torso, one in the center, and one light Gauss on each arm. Put a light laser in each arm, as you never want to get caught with no ammo remaining for your Gausses. Fill the armor as far as you can,
Use the following to create a Cauldron-Born 666 Mech. Strip all weapons and equip two Clan Gauss rifles, a Clan SSRM 4, a LRM 10, and an ER medium laser. Max out the armor and give it a LAMS. It may be slower, but now packs a wallop at all ranges.
Use a Daishi with 6 ER Large Lasers (Group 1); 2 Clan LBX-AC20s (Group 2); Components: LAMS; speed is 51 kph. The rest is armor. Try to fill up the torsos and arms and put the least amount of armor on the legs.