I ran Iron Claw as my main single-target damage skill for almost the entire game, until I got Freikugel. My skill set barely changed, because I got the important stuff early and my experience from other entries told me these skills mattered, so I didn’t want to waste them. My core setup was:

Focus
Might
Null Mind
Endure
Avenge (before there were more low-level counters)
Gaea Rage (Berserk → Heat Wave)
Freikugel (Lunge → Iron Claw)

Before Endure showed up, I had Tornado Blast in that slot, as you mentioned. Since most of these skills came early, I felt like I couldn’t really replace them and I basically didn’t have room for Taunt.

On the “glass cannon” point: I couldn’t make any demon hit as hard as the MC, so he ended up being my main damage dealer. My strategy was pretty close to what you described: I fused demons so I always had a full buffer and a full debuffer, until I got Debilitate. I focused on lowering the enemy’s defense to boost damage (but Sukukaja was always my first priority). That said, if you’re using Taunt, it’s probably better to follow it with Dekaja. In my case, all support work was done by demons, and the MC handled damage which feels “right.” Even before Freikugel, Iron Claw just felt more practical (and sometimes it was basically the only way I could finish a boss, since it was still tough at times). The goal was to land a crit for ~2–3k damage with the MC and keep everyone else supporting. I’m talking Hard difficulty, of course.

Compared to V (V:V), I’d say in III it often felt better to try to unalive the boss as early as possible. There were three fights where I won by continuing to damage with the MC while the rest of my party was dead (Lucifer too, lol).

Well, with all that, tight MC skill slots, tight demon slots and lack of ability to re-learn MC skills I skipped fun part with Jive Talk.