It would never occur to wizards to use muggle inspired warfare tactics. It's all a matter of reference frame, think about the cargo cult.For instance even though wizards are vaguely aware muggle weapons now consist or repeating "firelegs" rockets and explosive devices of all kind they possess only a very limited and summary understading for muggle warfare. The very notion of a mine field eludes them. Aurors for instance, are at best a paramiltary force, but mostly they are crime solvers and hit wizard land a hand when an extremely gifted and/ or dangerous high profile criminal/dark wizard is to be arrested Sirius Black for instance.
It's all about Wizarding culture and demographics. Unfortunately JK Rowling gave us no clear numbers, but I'd say GB wizarding population isn't over 50 000 individual and this is not a conservative figure.Think of the terrible consequences a modernday total war approach would have on the magical population, their number would plummet and the survivors better have ten kids per witch.
Wizards don't fight, they duel, resorting to artifacts other than the Elder wand or the "staff of Merlin" is unthinkable to them. There are muggle experts but they would not be consulted on such matters and muggleborns are seldom and their "polluting" ideas are seldom thought of as potentially useful. In brief all thinkgs non magic are considered useless or even dangerous. Think cultural identiy loss speech by pureblood supremacists.
The fact is they had a stategy, enchanted armors, gargoyles, Mandragoras and so on but they think the wizarding way. If they put their mind to it there a re endless possibilites the potential for destruction is trully astounding and dare I say it Awe- inspiring.Plus althought it is not specified in canon it's obvious Hogwarts iself was built by the 4 founders as a Fotress to harbour wizarding population in case of emergency.
Imagine Dragon dung fertilizer bombs, Erumpment horn rockets,spell controlled streams of garroting gas, blade ended broomsticks charmes to recognize their target with a picture like Prof Flitwick did in PoA so Sirius Black could open the Gates of Hogwarts.Or Scythe charmed to "clean" the battlefield... I can think or a thound other horrific ways magic could divert modern warfare ...
And I i'm not even metionning the terrible thing some clevely used spell could toimagine a vaccum dome or magic used to focus sun light and create a solar deathray.
And finally the whole purpose of defending Hogwarts was to Buy Harry some time until he could find and destroy te last Horcruxes, ultimately they were relying on Harry to end the war.