Lightscreen might prevent use of special attacks.. so it might not work well with mirror coat.. ah, I really don't know here. The only sure advice I can give here is to go with Thunderbolt, since Thunder usually proves a little too inaccurated without Rain Dance backing it up. If you've got Rain support elsewhere on the team, then Thunder would be okay. At any rate, Spark is not okay, since Thunderbolt is over 50% more effective.