Tech level has a whole lot to do with this.
Intro-tech. Yes you can try and run around and flank and kick undergunned and slow assaults. There is one thing often overlooked which I'll get to.
Star League/ IS Clan Invasion level, it gets dicey. Plenty of slow assaults left, but they are often better gunned than they were, more ammo etc. While the flanker units are too, those flankers still only have so much armor.
Clan and Jihad and after? Plenty of faster assaults that pack some really scary heat. The calculus gets impossibly tough. It is relatively easy to find some perfect flanker that can backstab assaults that can't cover themselves.
One point I think people miss at the Intro and SL level. Those flankers are often rather poorly armored. They often have just one weapon to plink away with. That WLF is not going to enjoy trying to trade it's armor against that Atlas, though to be honest people are making a near "Best in class vs Worst in class" (at least to deal with the problem being discussed) It's not a very legit comparison. That WLF is going to be one unhappy camper vs an Awesome. "Just get close" Sure, until you lose initiative and the Awesome gets to plop itself at 4 to 6 (best at 6) and melt you. Even something like a Banshee 3E, while being undergunned can rush a medium and make it fall back and potentially trap it against terrain or a map edge. Few lights would have the guns and armor to survive trading shots.
The other thing I don't dig about the false choices is that you don't often face an assault in this sort of a situation. The assault mech has friends. Those friends may well cover it's back. Even a BV Balanced game, a lance of Assaults would face 8 to 10 mediums, or a company of lights and Mediums. Those mediums MIGHT be able to flank and circle. OR They might see 1 of their number evaporate every turn. Sure, All PHX, GRF's etc can probably plink them to death, if your force is tuned to do it. And you know Awesomes don't Death Star you. What happens if you take to 2 PPC shots and fail that PSR? That Atlas might be in AC 20 range next turn.
I think people assume way too quickly that a fast flanker is going to down the assault. Atlas-D Sure. In a straight up fight with favorable terrain. If there is a mission involved? Perhaps heavy terrain that limits that move modifier, gives the Atlas cover or allows the Atlas to put their back to a building or a hill, or even a board edge? The Atlas has the armor to sit stock still and cut loose. If the Assault is a Banshee 3S, you can't dance and play. It will have the firepower to kill the flankers at range and up close and the armor to trade fire. You actually have to compare your armor and speed advantage to the armor and limits of the assault mech. If you are a PHX with a sad 128 points of armor and you want to use your 1 LL to drop an Atlas? That LRM 20 is mighty scary unless you can work behind the fatty.
I also reject that in play the "horde" tactic is effective consistently. Has it been done to me? Sure. Mostly through really well done combined arms with cheap but effective tanks, solid infantry and mechs that can come in an exploit things. Usually I got caught with a low mobility and high ammo force (In Mekwars campaigns you don't always have a lot of choice about what you have)
An AS7-D is 9.6 Million Cbills. 1897 BV. How do you want to balance the fight?
Tonnage? 5 Locust/Stinger/Wasps vs 1 Atlas. They are doomed. Not enough of them. 2 Centurions? They might win if they pull some sort of headshot, otherwise? That Atlas will get them. 2 Trebbys? Probably not enough ammo to drop the Atlas. 2 Griffins. IF the LRM 20 doesn't tear off an arm or leg and the map is big enough. They can probably take him. Course they are more expensive in Cbills and BV so by two measures that's not a balanced fight.
Balance by Cbills? so 6 bug mechs? No not enough. 2 Centurions or 2 Enforcers and something about 2.5 million Cbills? Commando or Valkyrie? Maybe? 2 Griffins are more expensive than An Atlas.
Balance by BV and one Griffin is on it's own with a rather cheap friend. 1897-1272= 625 BV left to take a friend. There are few mechs at 600 BV. Bugs are 4 or 500 tops. Most heavier lights or light mediums run 700. Atlas has a pretty good chance there.
It's just not a sure thing, or even likely to think mobility is always a killer against big slow assaults. It can be done, but it just isn't a simple as it seems.
I agree the OP was basically hosed with slow mediums vs big assaults. Either play for the interest "How long can the brave light/medium company hold out against the monsters?" Or Balance the fight by BV. If you can choose your forces, you can design a force that could plink the assaults to death. Unless they have Awesomes or Longbow 7Q's Or worse, An Awesome, A Longbow-7Q an Atlas and Something a bit faster like a Battlemaster or Banshee. Even a whole company is going to be pressed to down that.
Clans? Sure. Just bring fast units with LPL. Fun game that. Yes that will work. Unless they have Clan tech too and the right variant to match you or just flatten you with some monstrous ERPPC or mechwarrior nightmare weapon.