As above, but also use the highest quality socket that fits the plug. Some normal spark plug wrenches are a bit low quality, and whilst are perfectly fine under normal circumstances, not as good a fit if more force has to be applied.
Do give whatever you use as penetrating oil plenty of time, rather than use too much force and cause damage.
If as I suspect, the head is aluminium alloy, be extra careful.
Once the plug starts to move, that is just the beginning. Turn the plug both ways, use more penetrating oil and take your time.
