The best armour in this case is a good offense. It usually is the best defense against most scary things in DF.
Always have some crossbows available with copper bolts. A completely newbie squad of six or so will still usually rain enough bolts on a target to kill them. Good crossbows and bolts go a long way to even out lack of training. A horribly injured lasher will go down quite easily compared to a fresh one. They are not supermen, they just have super-weapons.
Also, bring the doggies. I can't emphasize this enough. The changes to the combat system has made it so that quantity of attacks is often superior to admantium "anything". The moment an enemy goes down almost every attack will be a dog chomping on head bits. And dwarves stoving in skulls with pickaxes, against next to no defense. Most return attacks will simply flay a dog, which is like free food and leather for you. Bring the doggies. Six to eight are good, fifty will make you absolutely invunerable to just about anything (OK, quite a few dogs will die, but they breed like rabbits in those quantities).
If axes, picks and spears can't do it, crossbows and dogs can. More is better as always, better trained is good but not really that essential. Armour is better than training (and lets them live to get training time). Three picks, three spears, six crossbows and as many dogs as you can spare and your fortress will be defended for life. Or FPS death.