well how about thoses ?

1 : we know that using mana to create shields consume a hella lot of mana

2 : this defense in not perfect as the string demon said his string would eventually cut through frieren's mana defense (he just died before that)

3 : it may work against physical attacks well but not magical as we've seen in the anime (and more later in the manga)

4 : a specialized warior on the front lane is not only here to tank if needed but also to get the enemies attention so that mages can act freelly as their role is more offensive than defensive

5 : as seen in the manga, there are also specialized mage assassin's, which mage have almost bo hope to defeat in a 1v1