Mage won't scales until about 10 levels in btw.
1-6, Geo is at its strongest with its Slow and oil field preventing enemies without gap closer from doing much.
Around level 7-10, aero and pyro are the best thanks to Epidemic of Flame and Chain lighting.
But after 16, Hydro-Pyro becomes the stronget become of Meteor and Hail shower destroying the whole area or outright freezing them all.
But for Sword-shield Warfare-Geo-Hydro with points in leadership after you get the skill unlocked is the best way to go. Don't forget to the Opportunist. BEing a tank, enemy will run through you to go to the back line and that is like, free AoO on enemy melee that run through you.
Mending Armor and Soothing cold are some of the best defensive in the game and you will love it. If you keep your gears somewhat updated on your squishies (so they have some armor) and fight the evenly leveled enemies, using MEnding armor + soothing cold along with leadership can easily keep your whole team alive.
And your source of damage will be shield bounce (unless you have a really good sword) and if you spec more into warfare, battlestomp and battering will be the two must-have skills because your job is not to perma CC and keep everyone alive as the tank instead of just soaking damage.
Necro is actually quite weak so don't fall into the same trap I did. You only need points to unlock skill in them. Investing further doesn't do anything since warfare is the one increasing its damage. If you plan no getting bonewidow, you will want to spec into Summoner as well but that is already way too much job for a single char.