Heal cures status ailments, such as Poison/Silence/Darkness.

Skill combos vary depending on your style, but I'll list a few.

Ramza - Samurai with Guts - if you get the Masamune from Elmdor, you can cast Regen+Haste on everyone nearby on your first turn. Pretty crazy. After that, you can use Scream (from Guts) to boost Ramza's Speed, PA, and MA. And these make a difference. Give him the Two Swords ability from Ninja or use Both Hands from Samurai to cause some ownage.

I gave Agrias Dance as a secondary once when I was bored - it had some good results. If she wasn't in Holy Sword range, then bam - Slow Dance.

Have a mage learn a bunch of good spells and then learn Calculator skills. If I told you one spell from each mage class to learn, it would be Holy (white), Frog (black), Sleep (oracle), and Haste (time mage). Imagine the effects of these spells if you had the right calculator ability. Frog and Sleep work astoundingly well to eliminate enemy threats.

Sometimes I almost prefer using Chemists to mages for healing - they can use guns (good range attackers), and their healing is instantaneous (items). Mustadio almost always becomes a secondary Chemist for me, and his Snipe abilities make him very versatile.

That's all I can think of from the top of my head. I had a ton of trouble beating Tactics the first time around, but it gets easier with time, because you know what to expect.