Haven't watched Fairy Tail in years and didn't make it to Fontaine in Genshin Impact, but here goes.
Immediately, Gajeel will outstat Tartaglia in every stat other than the latter's superior skill, which is even better than Fontaine Traveller, at least till Foul Legacy comes into play. which will bump his stats to be almost equal to steel dragon man (cuz Class M lifting strength > Class K). While he doesn't have an answer to hydro and electro manip (moreso hydro since I remember him fighting Laxus and tanking his attack or something), his Iron Dragon Scales defences will hold out well for him to shrug off his attacks. However, with amps, Tartaglia will be faster than him and his intangibility will help in dodging some of his hits. Ajax's long ass disease name I forgor will only come into play if the battle drags on, but when it hits, it's going to hit quite well.
What are Tartaglia's weapons made of? If iron or steel, Gajeel will eat them and replenish himself to be able to tank more hits.
Regardless of whether he's in his Foul Legacy form or not, Gajeel would usually be in his face, attacking him directly. While the latter can dodge and try to stay far away from him to attack him, iron dragon man outranges him hard. Either an Iron Dragon's Roar will be shot straight at him, he stretch his steel limbs into a weapon at him or just ******* pull himself to Fatui guy.
In Foul Legacy, Tartaglia has a better chance at going through his Iron Dragon's Scales and defeating Gajeel, but boy he's a tough nut to crack because of his amped durability. Even then, this form will strain him hard and will only last a while. If Gajeel hasn't already defeated him due to AP differences or Tartaglia hasn't defeated Gajeel before his transformation is gone, he's going to be screwed so hard.
Gajeel can probably take this fight mid-diff, but I'm still not voting yet in case of corrections and debates and stuff.
(Also how do match thread starters calculate the exact AP for the characters?)
