What rules are using for these tank-mech match ups? As Valadaar points out the rules for the old school battletech favored the mechs, the battletech rules that I am familiar also made tanks much more prone to damage to their drive systems.
Do you not the like the Omni mech system employed by the clans?
Even with the rules favoring mecha, I was routinely able to trounce my opponents with conventional forces. The simply fact of six tanks for the cost of one mecha was just more than the mech users could deal with. Tanks would be destroyed, and the tank building rules were a huge pain in the rear. Engines were massive, energy weapons required full heat sinks, power amps, and anything else that could be tacked on to slow down the tank. In the end, one heavy mech fighting six tanks armed with rail guns was simply a matter of time and the lucky shot that would take down the mech. The same applies for using bombing rules, VTOLs or anti-mech trained infantry. The mech and similarly the battle armor suits don't do so well.
Using mixed forces was the best defense against non-mech units. The best thing to deal with troublesome infantry is either more infantry, or light armored vehicles. Special role vehicles also did well removing their designed counterpart, such as the AA tank versus aircraft. But this is basic tactics and not any sort of advantage. The mech only worked because tanks weren't cool and compared to the 10+ tech manuals of mecha there are 2 tech manuals for vehicles.
So some real advantages in the rules: mecha need to be more durable, more agile, and need to be able to be more effective than their numerically superior and cheaper rivals.
Mecha have the initiative advantage, from their innate form of being worn, more so than being driven.
Lacking a central crew compartment or needing room inside for men to move around, the machine can be more compact and more heavily armored. This armor can also be shaped for causing glancing hits, deflecting hits and the like. In BTech the tank had unlimited weight for armor it could carry, while mechs were limited to a maximum amount of armor based off of their weight class. This system can be kept, but modified.
Nothing says a 50 ton mech has to have a chassis designed for a 50 ton mech. The 50 ton mech could have a chassis for 75 tons (this would be 7.5 tons weight versus 5 tons) and would allow the machine to wear armor for a 75 ton mech, or load itself out to 75 tons without concern for structural failure. The same can be done for the engine, mounting a larger engine than required by the basic numbers. This increased power could be limited for when the machine is unloaded, but could keep it moving at a regular pace when it is loaded. The weapon systems would only in certain situations be carried internally. The unloaded mech, unless it has internal weapons, is unarmed or very lightly armed. Thus all of it's build weight can be devoted to chassis, engine, and armor. The 50 ton mech mentioned above could have the armor of an old system mech 50% larger than it is, but retain the speed profile of a machine of it's dry weight. Depending on how much weight and gear the pilot wants to carry, the machine can be very heavily armed, and has a versatile payload potential.
Basically building reinforced chassis mechs and using basic encumberance rules.
Functionally, the new mecha would be omni-mechs, in that their weapons are removable and interchangeable. But by that description, we already have omnifighters.
