I'll respectfully disagree. The only time I believe a Merc would easily destroy a Marauder is if
1. Every knockback launches the Marauder off the ramp (Highly unlikely if the Mara is competent)
2. The Undying Rage, Force Camo, and Frenzy is on CD (Only after a close fight)
or
3. A knockback means certain death.
Every other time, the hardyness+ability to get back to the target with Force Camo+Frenzy+Charge gives me enough tools to beat a Merc.
Am I undefeated against them? Of course not, but I tend to engage them when I shouldn't. When I have all my CDs, a Merc is 100% dead.