I typically use support magic, with a majority of the damage done by trained units or my main Champion who sits in my Sovereigns pocket all day, However it is mostly likely, since I don't focus on mana, that my Sov is melee. Getting the best of both worlds. I like using Water and Life, but some times I add Wind into the mix (Any of the 3 in combination depending on the race, class, or playstyle). Sometimes the champion is a spellcaster, some times its an archer. I've even had a Sov Path of the Defender, while playing it as a mage with Life/Water/Life just to assist my ranger Champion, who got outclassed pretty quickly when a fortress became a Strike Tower...
It's the multiple element spells that really get me. Wind/Water Gentle Rain to be quite powerful in areas with low essence but you need more population. Water/Life Cloak of Oceans (?) to be useful for spamming mass heal/buffs/counterspell(Mana gain on the water one is great specially with other reductions)/thunderstorm. Wind/Life, the name escapes me, but it allows your units to act instantly after it is cast.
What supporting elements I choose tend to effect unit design. Going for glass cannon (metal saving?) style units Wind is a must, combing it with life practically allows for double damage (spears and ranged attacks draw no counter attack so its often 'free' damage), if the enemy is also slowed, and your champions are hasted, then expect many white wash battles. Add in life's 'shrink' then you can negate a large amount of physical damage to your glass cannons when up against a larger foe. Haste and growth your hard hitters (with some fire resist equipment or the earth ench if lucky) and you have a two man dragon slaying machine.
Something about using indirect magic tactically is that "magic immune" enemies wont hamper on your strategy too much. Since much of the focus is kept on casting spells at your own troops. And skills like fireball can be picked up by the Anointed Enchantments gained through research, should you really need them. You can also skimp on spell mastery and focus on spell resistance to avoid getting countered. Or cast the blue magical invuln spell.
Support allows for more adaptability should your situation not be what you were envisioning. Earth has some pretty sweet unit enchantments, but I prefer to bank my mana where possible, and regeneration can be a pretty handy 'after the fact' version of stone skin/cloak.
Then again death gets oppression doesn't it? It's like inspiration and enchanted hammers on a much vaguer scale! (Wail is a pretty nice non-shard nuke too I guess)
Too many walls of text, concision is really hard. 