i think it's much better to follow this path
Warrior>Infantryman>Gladiator>SM
because infantryman gives lvl15 Enhanced HP lvl10 Heavy Armor Expertise and lvl2 Troopership which give you enough HP to tank and Gladiator Gives Much better Crit and Damage Passives compare to Mercenary>Knight which the only benefit you could get is the lvl15 Heavy Armor Expertise lvl2 Solid Weapon and weapon and armor synchro.
but if you still want that path it is better to choose the
Guard>Merce>Knight>SM path