My suggestion is about making the fight longer.
And the only way to do it is by increase HP and lowering DMG.
Couldn't you also get a similar effect by decreasing accuracy and increasing action point requirement for better guns? This is by no means a perfect suggestion, but merely an example of another way to lengthen fights a little, or make them more of a fight than a "one click death" :
Increase action points needed to fire BG and rifles
Maintain action points for shit pistols
Lower action points for melee
Decrease accuracy at distance for all weapons (either lower percentage to hit, or cap skill at 150% or 200% or something)
Decrease accuracy of aimed / crit shots
Decrease accuracy of bursts at distance
Increase knockout chance for melee
So you have :
Smaller weapons fire quickly and accurately, but don't do much damage and have limited range.
Big guns take ages to fire, and are very inaccurate, but if they hit, they do lots of damage.
Sniper weapons take ages to fire, unaimed shots are very accurate and do moderate damage, aimed/crit shots much less accurate, but deadly.
Burst weapons need to be very near to be accurate.
If melee fighter manages to get to you before you've shot him enough, he'll knock you out and beat you to death on the floor.
To make things even harder, you could lose a few current Action Points, or stop for an "I've been hit" animation every time you are hit, giving a balance between "rubbish gun that fires often" and "deadly gun which fires rarely".
Alternately, you could have different levels of aiming (like in Jagged Alliance 2), i.e. each extra action point to fire increases the accuracy, so you can pepper them with 5 innacurate shots, or slowly aim and blow their head off.
I'm sure there's a million problems with those suggestions, and similar things will have been suggested and discarded before, but something along those lines would make fights last longer, simply by decreasing the amount of shots fired, and decreasing the amount of shots which hit - I'm sure increasing HP and lowering DMG can't be the only way to balance things. Though you say "this was tried already and whined about", there may be some compromise somewhere which works.