The Forged are bad ass...at lower levels. I used to get sent there on minutemen quests far too soon after rescuing Preston and its not fun. Good place to farm helmeted cage and spike armours if you're into that sort of thing though. Its not the greatest armour but it looks cool. Favoured weapons are flamers and molotovs so once you're hit you're fighting blind while the flame effect wears off. I'm not sure if energy weapon resistant armour will make much of a difference against them but if so its worth looking into asbestos for your armour and upgrading your vault suit. For even more protection hop in some power armour. You'll get a shishkebab from their leader and theres a minor quest to be found at a nearby farm settlement. Never seen a legendary Forged before so dont think there are any so dont worry about one of those spawning.
Raiders - pretty easy for the most part. You shouldn't be having too much trouble with these guys no matter where you find them. Places like Hyde Park and the Corvega plant have a lot of them but they're not the hardest of enemies.
Dunwich Borers raiders - Basically raiders in combat armour with some raider parts and combat rifles. For these guys you will want to kill the raider in the trailer at the top of the pit and use the terminal to shut down the turrets, then move across the top to the other side and kill the raider in the trailer there before creeping down the slope towards the bottom and mine entrance. Be aware there is one in power armour so if you see him try to get your critical hit on his fusion core and try to take him out early. This can also get you a power armour frame as well as the raider armour parts. Dont go here at an low level, theres no shame in running away and coming back once you have some decent gear. Although they're quite spread out in the quarry they're still pretty accurate at long range so if they see you then try to hide otherwise you're going to get gunned down from all directions. The mines are a good place for legendary ghouls.
Gunners - Higher ranked ones have decent health, most have pretty solid armour and a good selection of weapons including both energy and ballistic so you'll need an armour that can protect you from both. Fortunately they're wearing combat armour so you'll be able to loot them for some nice stuff. Keep an eye out for gunners whenever you need to upgrade to a new set of armour or new guns. Be aware that this faction has access to assaultrons. Theres at least 1 in Quincy. At higher levels this becomes an Assaultron Dominator or an Invader. Dont shoot their heads as it doesn't do as much damage, instead shoot their legs to slow them down. Dont try to blow their arms off to stop their melee attacks as they'll just kamikaze you.
Children of Atom - lightly armoured compared to other factions but they have a gamma gun. This weapon does a tonne of radiation damage (so while not damaging you directly it does drastically lower your max HP with each hit so you'll need radaway and stimpacks). You wont be fighting a lot of these guys, so far I've only found a crater with some hostile ones in it (not the quest related ones which are non hostile) and small bands of them holed up in buildings being attacked by gunners. You can sit back and watch the fight then go in to kill and loot any survivors. Personally I dont like gamma guns, some people do so give them a try and see if you like them or not but ammo isn't exactly common as the stores dont sell a lot and CoA are quite rare themselves. If you do like the gamma gun its still not a primary weapon, especially since its not great against ghouls or mutants. If you insist on hunting these guys then get yourself a rad suit. I dont think you'll take as much radiation damage but the weapons can cause a lot of stagger.