Mole grips will be too small for large bottle caps. The jaws on my 9" pair only open to approximately 1 1/2", and are not deep enough to grip round a large cap. The sizes refer to the length of the handles, not the jaw opening size. Trying an adjustable pipe wrench, the handle grips would probably be too far apart to hold in one hand. A 'monkey' or pipe wrench might be a possibility, but could be too heavy for your hands:
Amazon.co.uk User Recommendation
Otherwise, one of the Arthritis charities may have suggestions of the best utensils to use, especially when you have to also grip the bottle as well as the cap, while applying a twisting motion.