Thanks a lot for all this info, I'll try some other strategies the next time I replay the game.

But this is undoubtedly a bad game design decision and no other boss is as hard as this one. Why the first boss is the hardest one in the whole game is beyond me.

I played this on the hardest difficulty setting, but with no mods. I also had trouble and sought advice on the internet, but once I figured out the best strategy, it's not too bad. Here's what I learned:

1) You can knock the beast down with Aard, if you've leveled it up. As a barghest, he is susceptible to knockdown attempts. When he's knocked down, you can one-hit kill him. Unfortunately, I had not leveled up my Aard at this point, so this did not work for me.

2) You can use the strong style against the beast, even though it's not effective against normal barghests. The strong style does much more damage.

So what I ended up doing was, I took a swallow and a blizzard right as the fight started, and then I just used the strong style against the beast, ignoring all the other barghests. I had to be careful because with a blizzard potion it's harder to time attacks right since things are going in slow-mo. I had to be sure to actually chain my attacks without missing any. This method steadily pushed the beast back, so eventually the barghests left me alone and went after Abigail (I had decided to save Abigail) and I was left alone with the beast. He would occasionally inflict pain on me, but the swallow kept me alive and then I resumed beating on him. Eventually he went down, and when he goes down all the barghests disappear.

Also, don't worry about keeping Abigail alive, it turns out she can't be killed in this fight.

Oh my god I hate this fight, haha.
The first time I tried this I'd luckily put points into Aard so I was fine..forgot to this time. After about 20 tries and following some of the advice here, here's what I did step by step:

-save at start of the fight (immediately after cutscene)
-pause
-down Blizzard
-run to the other end of the path
-draw sword as you're running (make sure it's set to Strong style before your save at the start of the fight)
-down Swallow
-use Igni on Barghests when they come in close (if it's maxed, if not, Aard)
-attack Beast only using Strong style
-timing is tricky with Blizzard, but basically as soon as you hit the ground, click again...if you mess up, you'll probably have to...
-run around for a few minutes until your hp is full or nearly / use Igni (or Aard) again
-when you're good, attack the Beast again with Strong style
-repeat attacking/running/using signs until he's dead
-save immediately (another fight coming)

You may want to throw some Specter Oil and/or a Tawny Owl in there at some point, but I didn't.

The rewards at the end make it almost worth it..almost...

I was surprised by that too, but it turns out that the plant is actually one of the special enemies that you can kill and then bring their heads back to town for a reward. So it's an extra-tough version of the normal Archespore plant (which is also why its name is Coccicidium or something, instead of simply Archespore). That confused me for a while as I thought it was some different type of enemy that I didn't have in the Bestiary yet so I avoided it for a long time. It's a better idea to just kill it.

MINOR SPOILERS:

The key to killing it is to set it on fire. Potions that protect from poison are also helpful. Once it's on fire, if you have the talents that make the Strong Silver style do extra damage on a burning enemy, that helps too.

I was thinking that. Fire and more fire. I wasn't sure was it acid or poison what that SOB spitted. So next time I play I'm gonna sit down and brew some potions. So there is no book for that? Well, good to know. That was indeed the same enemy. =)

I use a torch when gong into that cave, provides lighting and makes for an excellent weapon against the plants too

The bestiary entry for Archespores indicates that they shoot poison thorns. So I think it's poison rather than acid, but I'm not certain. You could experiment with a Bindweed potion (protection from acid) to see if that helps, or alternatively a Golden Oriole potion (protection from poison). You might not be able to find the formula for Golden Oriole until later though.

If you have the Igni upgrade "Incineration" and the Dexterity upgrade "Deflect Arrows", it gets much easier to eradicate archespores. (If you can reach only one of these in Act I, take "Incineration".) In Act II, you get a silver sword; take the "Patinado" upgrade to Strong Silver too.

I hadn't thought about deflect arrows being used for archespores... do you need to have your steel sword out for that to work? My understanding was that steel swords are used for parrying but with your silver sword you rely on dodging instead.

I don't think deflecting arrows requires the "parry" mechanic.; I seem to recall it working well with a silver sword. I'll try it again tonight. Anyway, for the Act I echinopsae, you have only the steel sword to work with.

I think I have deflect arrows skill but not many archers around. So it automaticly deflects arrows when I have a sword in hand? I don't remember what I have in firemagic. Propably many things. =) I'm taking my time with the game anyway, no point just rushing the game forward. Golden oriole and bindweed recipes I do own. I think I am in act two.
Weird thing happened the otherday. I could buy stuff from the armorer of flaming rose dude, because of siegfried. (well, I didn't buy, I didn't have money enough).. But when I later tried to buy or look the stuff, it didn't let me. Well thaler sells the better jacket also. perhaps I'll play some dice.
It's not that i'm stuck or something. Plenty to do.
It was nice to see some concequences for my actions. Last time I played.. months ago, never have finished the game. I killed the elves who came to take the stuff in the crates, those stuttering fellows crates. This time I didn't so coleman died and I was like "should have bought the fisstech when I had the chance" =)
And gerald is getting plenty of sex in the meantime.

Yeah that happened to me too. I think it might be triggered by talking to Siegfried again, and he asks you to join the order but you refuse. Anyway, you can go to the dwarven blacksmith in the nonhuman district instead.

Also there are other ways to get fisstech even if Coleman is dead.

The armorers (Order armorer, Dwarven blacksmith) are very choosy about doing business with you, and there are several choices you can make that will turn them one way or the other.

If you sent Siegfried away in the sewers and fought the cockatrice on your own, the Order armorer refuses to deal with you.

Refusing to join the Order doesn't affect the armorer, but clearing Vivaldi (which you may have done early in Chapter II) also turns him against you.

One cheesy way of instantly killing the best would be maximizing your Aard sign intensity by using the Place of Power and performing the Ritual of Magic before entering the Salamandra Hideout. You also may be able to make a Petri's Philter (even though you don't have the recipe yet) with a White Gull, two Rebis, one Quebrith, one Hydragenum and one Vermilion. The only problem may be finding ingredients with the last two, but I seem to remember looting at least one from some abandoned house and I think Abigail may have some in stock as well. You may also be able to clear out the first floor of the Salamanda Hideout and then use the Aard Circle of Elements to the South and might still have the buff by the time you fight the Beast if you're quick enough. If everything worked, you could get a 200% intensity bonus to Aard.