My tactics aren't perfectly flawless, i don't take any smash bros game too seriously. but i'll try, i recently grew a keen interest in marth, and my brother mains ike and we face off all the time.
Marth generally is faster, so you'll need to use your strength to beat him, marths dash attack is pretty good on the grounds that it's fast and unexpectable. If you get in a counter for this, good.
Marths shield breaker (neutral B) is pretty lethal, if you don't dodge it quick enough, you'll be broken and marth will do as he pleases. unless you feel really confident or can see into the future, Don't counter, unless it's in the late stage where it will force-start, that's after the fourth noticeable charging sound marth makes when he charges it. you'll be better off getting it then.
Dancing blade (side B) is dodgy, because he can go three ways, multi-stab (Hold down before last button press), ordinary knock back or flung in the air (hold up before last button press). you shouldn't need to worry about about the first two as much. Flung in the air can lead to air attacks, an up smash if you poorly time your anti-air plan or a dolphin slash (marths up B). possibly the 3rd one if he's getting you off the side.
Marth can counter (down B, ike can too) every attack (unless someone corrects me, seeing as i am on the ike board) ike can throw. aether (ikes up B) is the worst because he can counter in the air, combined with dolphin slash, marth uses these two to ensure ikes feet never touches ground. you can only pray that marth screws up and gives you a chance.
Dolphin slash is the only attack you shouldn't try to counter unless you have the high ground, shield it instead, if a platform is above, you can hopefully up-air him. unless marth is recovering, and screws up grabbing the ledge, you set yourself up for a F-smash, ikes killer move.
Quick Draw (ikes side B) works too, it isn't forced, so you can catch him off-guard with it, eruption (neutral B) should never be used unless marth screws up horribly, seeing as i know no advantages of it. Counter will be your savior if you wish to avoid marths super moves.
As for his Final smash, don't stand in front of him, ever,
That's all i can think of without going overboard or stealing from ryko. Unfortunately, i don't tend to know all of the hip young lingo these smash players use anyway, so peace out.