I actually consider Str/Int to be the best of the hybrids, but it's mostly due to Shields. Shields offer a strong defense against attacks regardless of whether you're in weapon-mode or caster-mode, and thus multitask extremely well; attacks are also kind of your weak spot, considering Weather the Storm and plate armour are going to make you extremely resilient against spell damage. With these strong ability-based defenses, you don't mind the attribute spread (specifically, the Con loss) from going hybrid nearly as much.
The issue is that incorporating shields into your game plan means you don't really have room to do 3 schools of magic properly.
Personally, I'd recommend...
Man-At-Arms 5 (for Weather the Storm and Picture of Health)
Aerotheurge 5 (for Lightning Rod very useful on melee, you can ignore static clouds and electrified liquid)
Shield Specialist 4
Pyrotechnic 3 (for Explode, you should be able to get your Fire resistance to the point that it heals you, like an AoE Vampiric Touch)
Tenebrium 3 (only moderate investment because it's useless when you're casting spells)
Total points: 52 (49 for level 20 + one ability book)
I'd go drop a point or two from Tenebrium/Shield, and get 1-2 points in Hydro/Witchcraft/Geo. Bless (Geomancer) and Oath of Desecration (Witchcraft) are especially good for hybrid mage/warrior builds.
