So, like the title says, create a combination of several elements from several different armies (in the same era) to form a (nearly) undefeated, new army...
And ignore logistic, terrain, numerical advantadge, etc...
For ancient age, I would say Roman legionnaire + Macedonian hetairoi (companion cavalry) + Parthian horce archer would form an invicible army, even Han Chinese army will have a hard time defeating that...
And as my knowledge about era outside the ancient age is VERY limited, can someone think what kind of combination needed to create an undefeated army, for middle age, renaissance, imperial age, industrial age, WWI, WWII, and modern age...?