This would not work. Mold Breaker prevents the user's attack from being negated by abilities (For example, Rampardos can hit through Wonder Guard, or could hit through Flash Fire with a Fire-type attack).
Rampardos doesn't take influence from Slaking's ability, and since Slaking's problem is a lack of action, rather than an issue with attacks connecting, then this would not help pick up Slaking's, well...Slack.