Well, in my headcanon, Tia was holding back when she fought dem bugs; I think her using full power would probably cause a nuclear-size blast, killing everypony in Canterlot and Ponyville.
Fights between two airborne groups would probably be much like dogfighting between jets in the air today.
I think Equestria is in the modern era in terms of technology. According to the CiV G+K tech tree (http://www.minosha.com/civ5/techtree.html), Equestria has many modern technologies, but almost no Atomic era techs.They have railroad, as seen by the train to Canterlot; flight as seen by Twilight's hot air balloon; replaceable parts as seen by Rarity's sewing machine; plastics as seen by the many plastic containers throughout the series; electronics as seen by Twilight's basement gadgets/Rainbow Dash's movie in Hurricane Fluttershy; and combustion as seen when Dash names her turtle 'tank'. However, they don't have penicillin, atomic theory, radar, or combined arms - so they're in late modern era. This means that they have artillery, infantry w/ pretty advanced rifles, machine guns, battleships/destroyers, bombing squads, and primitive tanks.
I think unicorns on the other side would find ways to counter that spell with their own magic. Also, I'm sure there are certain spells off-limits in battle to even unicorns. For example, it is said in Hearth's Warming Eve that hundreds of unicorns can raise the sun, so what prevents them from doing the comparatively easy task of harnessing a nuke's worth of atomic energy and unleashing it upon their foes? Surely such spells must be illegal even in battle.