This wave is called wave 13 (unlucky number for some people) for a reason. This is the very first wave you encouter Spiders. Both Leopard spiders and Normal spiders appear on this wave. Get bannermen because you will need them. Riflemen on the walls is also a good choice. Kill the spiders quickly. Kill the leopard spiders even quicker. If you can, use a ranged character with very good attack, staying out of range of the spiders jump attack. Push the ememy back with Wood Elf Warriors until they are about in the middle of the map. If you pushed them back but are still struggling, buy a few Elven Hunters to kill them from afar. Ranged attacks are key in this wave.