First off, remove the road wheel, don't struggle trying to work round it.
Always support vehicle with an axle stand and not just the jack.

- Wiggle a wire brush in the phillips heads to remove crud before attempting to undo.
- Use a good quality unworn well fitting screwdriver
- If stuck try tightening a little first and then revert to undoing

- If all else fails carefully heat the head of the screw up with a heat-gun and pull the arch over the screw head, once cover removed you have more room to attach a pair of mole grips to remove the screw

If removing liners in cold weather they easily break so also waft a heat gun round in the wheel arch to soften them up slightly,it makes all the difference