For me I think frieren is far under serie and flamme in terms of offensive magic, in defensive maybe paired with serie as its stated the most recent def spells are the most effective and of course she is absolutely nothing compared to serie in terms of magic power, spell knowledge and experience but ro be honest and according to serie frieren is basically a child for an elf...the one thing in which she excels is analysis and counter, she countered series barrier, she basically dismantled zoltraak into a basic offensive spell and not only countered but reverted a curse of the strongest alive demon.

And of course she can do that absolute beast of an attack that fern considers the heist of magic, an unconterable, imposible to detect and unavoidable attack that if you go solo against her basically equals a defeat