Which tool for fixing wobbly crank? Any idea what kind of tool should I but to fix the loose left crank on my bike? I found this https://www.amazon.co.uk/Park-Tool-CCW-5-Crank-Wrench/dp/B0012Q5XAS but not sure if it's gonna suit. Or is it enough to buy only nut driver?
The Park tool will work. A 14mm socket and a ratchet will also work. If the crankarm  has been loose for some time there is a chance the movement has ruined the crankarm by distorting the square hole. I would attempt to tighten the bolt then see if the arm still wiggles on the crank axle. If there is no movement, ride for a few days then recheck the bolt tightness and for wiggle. If everything is good recheck in a week. then after 30 days. If everything is still tight you should be all set. It is a good idea to check the drive side while you are at it. As the bike looks fairly new I would recheck all the hardware, stem and bar clamps, pedals, wheel QR etc.