Yeah copper bolts should be sufficient against gobs. As are often bone bolts, but if you have surplus copper on your map, make copper bolts.
Dwarves are stupid. They will not follow orders exactly and as soon as they see an enemy they'll never respond to your orders until the enemy is dead. When you design your defenses, you have to take that into account.
A very simple solution to shift the odds in your favour would be to construct a couple of bridges so that you can split the enemy into smaller groups. This way you can focus all your killing power on a few enemies and if some of your dwarves get injured you can recover them and depending on the status of your military lower the next bridge and keep fighting or let them rest up.
Also: make a staging area for your soldiers
out of line of sight to the enemy, so you can group up before you head into battle.
If your equipment is top notch (and it is, apart from the wooden bolts) then the fault must lie with your soldiers.
Traits like agility, willpower, toughness and strength are very important for a soldier:
- if half your army is 'flimsy', they will suffer injuries
a lot- no agility means very slow dwarves means often times death; note that agility influences the general speed of a dwarf, not only his movement!
- no willpower: they give in to pain quicker which is basically one step away from being killed
Marksdwarves are somewhat easier to handle in that regard, but your melees should have at least a few of these traits. Also of high importance is "quick to heal", obviously. Having your frontliners
'cross train' can increase stats.
Other than that, look throught the wiki and search the forums about improving the dwarves' training, since, as the others have said, skill is more important than anything else.
As ultima ratio (heh, always wanted to use that... I'm smard now) there is also the danger room - its effectiveness can range from outright exploity and cheesy to mild and passive skill improvements. Everyone has a different opinion whether it should be regarded a legit way of training or not, you'll have to see for yourself if it floats your boat.