Dungeon is an offensive faction, yes, but look at Attack skill & perks: only +15% to melee damage at expert, Battle Frenzy is useless for your troops because they are low in numbers,
but with tactics, you have 3 very good melee units that can cross the field and do damage in one turn (lizards, maidens, and dragons), and the maidens can come back from that (so defense not an issue).
adding frenzy to your maidens works quite well.
I think if you can find something to boost initiative, offense makes dungeon an excellent first strike faction.
however, inferno also has units with better initiative that can cross entire field in one turn (cerberi, devils, and nightmares).
so, in that case, then yeah, defense would be the better choice.
so, it depends on the circumstances.
i personally HATE shooters, so i pick offense for dungeon so i can get my maidens and dragons in their face on the very first turn.
however, if i expected to be mostly fighting inferno, then defense probably would be a better choice.