It's true that you're outnumbered and they're the same level as you, but if you focus on using Area of Effect powers to hit multiples spiders at once, and maybe also hit spiders and adjacent TNT barrels at the same time, it's perfectly doable. Also, try not to run away unless you have a good reason. The spiders have Cheap Shot, and can get to you in about two turns, so it doesn't really preserve your health.