The downside is that it can take a long time to get an army skilled enough to a point where you can take goblins head on and not take losses. Here are my suggestions.
1. Garb - Ensure all your melee troops are wearing the best possible armour your fortress can produce. Start out with iron, nothing less, all high quality full plate with shield. If you don't have a highly skilled armourer, train one up by forging -> melting -> reforge if you can afford the resources. Once you kill a few packs of goblins, I suggest melting down their narrow steel and using it to forge new steel armour if you can't produce it yourself.
2. Skills - Ensure that your troops are highly trained (i.e. "Great" or better), that includes wrestling. The problem it that it levels really slowly when wielding a weapon so I suggest training all your dwarves in wrestling alone while wearing heavy armour (incl. shield) before starting them with a weapon.
3. The approach - Digging underneath goblins is always a good idea if you want to launch an ambush. If you can't be bothered with digging, you can safely position your dwarves about 1 or 2 screens away without them noticing. I suggest putting all you dwarves under the command on a single dwarf. You also can lessen the impact of arrows / bolts if you approach from trees or hills.
4. Royal / Fortress Guard - It keeps your Nobles from becoming unhappy if you have enough guard. After several seasons, you may find your fortress / royal guards quite well trained and you might want to swap them out into your regular army. Note that if you swap out a dwarf from the royal guard, s/he will get an unhappy thought for "being dismissed from royal guard".
5. Siege weapons - ballistas are much more effective than catapults, but I believe they need to be on the same Z level.
From personal experience, a few dwarves with the high quality steel armour and legendary skills can take out packs of goblins alone. However, you can lose any dwarf to a lucky arrow or bolt so it's always good to mix in lesser recruits when you go out to battle.
Here is one of my vids showing you the fun you can have with a top notch army.
Good luck!