The Terran Marauder

Against The Zerg

Weak Against : Brood Lord, Mutalisk, Hydralisk, Zergling

OK against :  Baneling 

Strong Against : Roach, Ultralisk

Against the Protoss 

Weak Against : Carrier, Mothership, Void Ray, Dark Templar, Immortal, Zealot 

OK Against : Colossus, High Templar, Sentry

Strong Against : Stalker 

Against the Terran

Weak Against : Banshee, Battlecruiser, Siege Tank

OK Against : Ghost, Hellion, Marine

Strong Against : Thor,Viking,Reaper 

The Marauder is a basic  armored infantry unit in the terran army .He is very strong in large numbers against structures and armored ground units.The downside of the marauder is that he can atack only ground units,leaving him vulnerable to air units. Terran players commonly use marrauders paired with marines (aka M&M bioball) or with marines and medivacs (aka  MMM)

Upgrades

• Stimpack : The Stimpack upgrade works in the same way for the marauder as it does for the marine,it gives a 15 second speed and range buff at the cost of 10 HP.Everytime you use the stimpack your unit looses 10HP but the benefits are great for large groups of marines and marauders.You can easily kite larger more slower enemies ,or you can use it for a quick escape if things don't go in your favor.It is best to use the stimpack at the start of an engagement as you will make the most fo the ability's durration.Also keep in mind that the stims do not stack,if you cast it more than once you won't get additionall buffs,you will only loose hp.

• Concussive Shells : Reasearched in the Baracks techlab along with the Stimpacks,this upgrade is a must for marauders.With this ability the marauders slows any enemy he atacks for 50 % of their movement speed, great when facing a large zealot army,or reaper harras.

Using Marauders Against the Protoss

Marauders are a great choice against most Protoss units,they can  easily outnumber and destroy formations of Zealots and Stalkers.With proper support from other units they are good against Immortals and Collosi.The units to avoid with marauders are Void Rays and Carriers,never engage an air army if you are heavy on marauders.

Using Marauders Against the Zerg

Marauders excell at killing Roaches and Hydralisks,with concussive shells being especially deadly. Be aware of zerglings with the marauders,they are very weak against packs of zerlings on their own,but can easily handle them with a marine or hellion escort.Because they can't atack air units you must avoid mutalisk formations at all costs.Against mutalisks they are better paired with Thors and Vikings than with Marines.

Using Marauders Against Terrans 

As always T vs T matches can be a little tricky.Scouting is a vital component in matches like this.Against terran marauders are very good for early rushes ,before your oponent rolls out siege tanks.They can be amazing against heavy mech armies,such as thors and vikings .With proper micromanagement and support they can bust thru a siege tank fortress,and with the help of stims can destroy formations of tanks before they go into siege mode.