You are correct, sir.For the time being, though, it sounds as if tactics and planning are the problem, not the actual make-up of the characters. You can help by giving them encounters in which the monsters demonstrate tactics that the players might use next time. Let them win an easy encounter where the monsters use a doorway to achieve bottlenecking - that might give them the idea of using bottlenecking themselves in another encounter, particularly if you talk about it afterwards. Give them an encounter where the monsters exploit stealth, talk about it afterwards and get the players to think about how they can use stealth themselves. Make sure everyone has read the rules about Stealth and Hiding in the PHB!
Did the players pick race/class before or after they rolled the stats?

It also means that buffs that add a +1 or more are MUCH bigger than 3E and feel closer to AD&D in power. Three members of your party swinging melee weapons with Bless will be doing a LOT more damage over the life of the combat than that Cleric jumping in immediately and swinging a mace. It will also end the combat fast enough that you will spend fewer spells healing later.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.