One blow and I'm dead. I have tried casting all the signs and survive by running around in circles but am unable to approach
him from behind and he swings that club in an arc so even from the side I die.
Is there a trick that can be used to kill him as I am unable to proceed until he is dead?
For the record, this quest is four levels lower than my current level.
First play through easy. This NG+ has many enemies up scaled by plenty.

You need to upgrade your Quen, to get the protective 'bubble' that even heals you when you get hit.

Are you talking about the Golyat that you encounter as soon as you get to Toussaint? If so, there's an easier way to kill it if you want to try it: simply hit him in the eye with a single crossbow bolt and he'll go down. It may take a few tries since he wears that protective helmet and moves around a lot, but all it takes is one well-placed shot.

I did upgrade quen but only on occasions does the bubble appear.
I found that using Northern Wind bombs froze him allowing me to get in one hit but no more.
I died over a hundred times - luckily I did not have to sit through the normal cinematics that often precedes
a boss fight. Eventually I discovered by NOT keeping my distance but keeping close to him, dodging and moving around him
he eventually died and I took little damage.
I still have to face The Caretaker in that Hearts of Stone quest. Not looking forward to that encounter. Serious? The crossbow bolts are all rated at low levels. For some reason they did not up scale crossbows in NG+ like they did to swords and armor.

You need enough stamina, which you should always be watching in a fight.

Having killed Golyat I thought killing the Scurvers at the river would be simple yet it took me ages.
Then getting killed facing that vampire, and finally Shaemaar. Playing NG+ in Toussaint I feel that
enemy levels at normal level have been up scaled more towards the highest level.
What's more, all side quests I have received in toussaint are rated at higher levels than my current level.
I still have side quests in Velen and Skellige but they are now about 20 levels lower than me so it's pointless
doing these quests then receiving one XP point after completing the quest.

From all of your posts it sounds like you're not having very much fun. Honestly, I don't know why you even bother with NG+. When I did try it out, ages ago, I gave up before I finished White Orchard; I hated it.

That's why I take tawny owl with thunderbolt.