My question for the OP is: What force are you putting together to defend against the big stuff?
More specifically, is this a force you're putting together to stop your opponent, or is this a force already existing that you're in command of? If you have a choice, yeah, the above tomes of advice are good advice (speeeed!). If you're being told that this is the force you have available- a garrison force defending against an attacker, for example- and you don't have any changes you can make, that's different. In war, you don't always get a chance to field an optimal force against the enemy. (My granddad had some stories about Bastogne, for example...)
Assuming you get to build a force, yeah, what you have just doesn't have the mobility to do the job- you're trying to take on Mechs twice your own weight in a slugfest, and their armor simply makes that a losing proposition. Speed is life against heavier opponents- if you can get it, do it. Spiders are a longtime favorite for fans, because it moves like a ferret on meth, but your bare minimum should aim to be 5/8/5. Jump jets are a key, to me- be able to disengage if the going gets tough. That means that while units like the Panther or Enforcer are still useful, they shouldn't be relied on as your centerpiece- they're just not able to change positions enough to ensure their safety. 4/6 units without jump jets (like the Centurion) are as good as speed bumps- at least aim, if you can, to get long-range variants of those Mechs out there so they can fire sooner before being overrun. My advice for units to explore include the Phoenix Hawk, Clint (seriously), Trebuchet (if you have a large map), Jenner, and Griffin.
If you're just stuck with a force with no alternatives to swap out because that's what's on hand per the scenario/campaign requirements, that's rough- your opponent has huge advantages here. Terrain is always helpful, but with your speed issues he has the same advantages you do overall. Your best bet is to group-fire on one or the other (preferably the Crab) and hope you can put it down with overwhelming force before dealing with the other one, but that's a rough proposition- better than any other ideas that come to mind, but still not fun. In war, sometimes you have a losing proposition and can't do more than your best, but the force you listed above is in for a rough day- which might be realistic, but not all that fun as a game. If they're on the attack, see about the possibilities of putting down minefields? In particular, vibro-bombs mean your lighter units can walk right over the top of them, while his beefy monsters are finding the ground exploding with every step. They honestly don't do a great deal of damage usually, but if he gets paranoid about where he's walking it might give a little more of an advantage. Thunder LRMs are a later invention, but also handy- he can see where they land, but that can be handy itself- he knows where he can't safely walk, and watches avenues of advance slowly disappear.