some good defensive pokemon in sinnoh include Magnezone, Togekiss. Golem, Gastrodon, Bronzong, ect.
But what I would do for your team in particular is I would probobaly switch Sceptile out for Assualt Vest Tangrowth
Move 1
Giga Drain
Leaf Storm
Move 2
Knock Off
Move 3
Hidden Power Fire...