Serp over slowbro, weeelll slowbro is defensive and serp is offensive, so you cant judge it, unless the slowbro is offensive, then serp is better (though you'll get another birdspam weakness).
As for manaphy, chansey is your best option. Also, and fast pokemon (over 100 base speed) and a super effective hit can ko it with some prior damage (100/100/100 bulk is nothing to laugh at)