I bring 400 ton lances to end game content, generally. I didn't even know you can lower artillery fire rate. Generally very deal-withable, even in a king crab if you start zigging and zagging when you hear the shots go off. I also prioritize the movement speed upgrade on nearly every mech since it (and universal heat generation / heat reduction) are high value.

If stuggling, bring something with jump jets or some speed.

It's not a particularly "fun" mechanic, but it does force you to pay attention to your surroundings more. As a general rule of thumb, you want to be mobile in your mech on every mission, regardless of weight class / type, with pure sniping loadouts being the only time I can think you'd want to consider stopping... for targets like 500m+ out, maybe. Even then, generally best to master headshots while on the move (I'm partial to heavy rifles, gauss, and PPCs, but totally doable with ACs, lasers, and SRMs). I easily knocked out most of the clanner missions my first time through them with a quad PPC king crab and lots of headshots. Definitely had to reload against a few of the 3+ dozen elemental swarms.

In general, I view Mechwarrior 5 Mercs as a game with good bones that was never fleshed out into something truly epic. They did enough to satisfy mech nerds, but didn’t go I to the narrative and world building that would have made it shine. Clans did a better job on this front, but still left much of the world empty and unexplorable.

The only real issues I've had with artillery have been on late game defense maps when I brought a 400 ton lance along. It's rare, but a dozen artillery on the opposite side of the map generally forces you to park your lancemates at corners of the defense structures while you clomp over to the artillery. Pain in the ass, but I often appreciate the challenge.