Sorry if you already do this but are you removing a weapon on each initial mech so you can increase the armour a lot?
Some do not realise each mech is highly configurable including increasing the armour, and increasing the armour on each mech is incredibly important to the point it makes sense removing certain weapons to allow it.
Also move ammunition to legs as it is less likely to explode there due to way targeting occurs; only reason to leave it in torso is if playing closer to original lore.
Problem with ammunition is that it has a high explosive damage count against the mech, and once down to internal structure armour only in that location can be likely to happen.

Also it is very important to know how you want to increase the skills for each of your mech warriors in the lance;
Glitch I focus primarily on Tactics (looking to get to Called Shot Mastery) and Gunnery to 5 or 6.
Behemoth I focus more on Guts (for Bulwark and higher then to get heat threshold increase and recoil reduction) and Pilot to I think level 6 so has a bit better evasion and Sure Footing (has an additional evasion as part of that ability).
Decker I focus on Pilot and Tactics.
My Character; various ways to go and depends how you visualise playing that but I like Tactics and Guts (Vanguard).

There are guides out of date relative to 1.8 but can still be useful on Steam - I cannot posts links yet unfortunately.

just to be clear, moving ammo to the legs is only a good idea against the ai... don't do this if playing a human

One thing that I did in the early game (and still do) is have 1 pilot trained enough in tactics to get Sensor Lock as a scout. Keep him in a fast mobile mech (the beginning Spider works).

Keep the rest of your lance somewhat together at the beginning of a mission. Have them move as far as they can where they will be in the cover of trees, and preferably out of sight of the enemy. For your scout, move him close enough to the enemy where he can stay behind cover, and use Sensor Lock on the enemy blip. It will essentially mark that enemy for attack by any of your other mechs, as long as they have a weapon that can hit at that range. This is perfect to send out LRMs to slowly whittle away their armor. Keep that scout out of range from being attacked while you position the rest of your lance for taking out the rest of the enemies. It takes a little longer for a battle, and you have 1 mech that's not as useful in dealing damage, but it definitely helps when you can attack, and your enemies can't.

If facing something like a Hunchback with an AC20, keep your main group as far away as you can and in cover (jump jets help), and hit them with your long-range weapons. As long as your scout keeps pinging them with Sensor Lock each turn, you can attack. They might not be able to, because they might not have line of sight to you.

The other bonus of using Sensor Lock is that it removes 2 evasion from the enemy. If you have any mechs that aren't equipped with LRMs, I generally have that pilot also have Sensor Lock, so they can double up and remove 4 evasion from the faster enemy mechs.

Yes, too it's usefull as aggro tool, being sensor locked is a provocation and AI wants to kill our hidden scout, almost ignoring our other mechs, so you can make a trap with them.

In the end, the missions where I suffer the more, are those where we have to set 3 beacons for a satelite strike.
There are so much enemies in same time from every where (we must have a good rear armor), and as we can't destroy them all we have to flee.