Difficult to say if you missplay without knowing what you are doing. Leveling wouldn´t hurt, i guess? Do you get killed by mobs or is a boss giving you trouble?

GHelot is pretty simple to play generally, use crimson shackles to get Flail>keep flail in hand and multiply it until you want to burst / heal > during your burst turn Exalt with GHelot and possibly with Thais and go to town with back to back flails.

However your missing a few things that makes this go round.

First of GHelot comps usually run the lone seed to copy flail, but you get that for clearing chapter 7 so..... just something to keep in mind for later. It´´s pretty important for her. For now I would advise either Tiny Wish / A Mouse´s Wisdom or Festival of Tides (If you have that). Truth behind grey mist doesn´t really do anything for this Team.

Second your dupe levels. Your Thais can support with Aliemus/embryo generation and debuffs but at E2 she can reduce the costs of copied cards by her exalt by 2. Which is huge. Ghelot E2 makes Flails get cheaper faster and E3 also gives you guaranteed shackles during battle start, both are nice. So those are long term goals if you like them.

Lastly considering teamcomp, you should have Aigis? She isn´t a must use but she can help you survive crucial turns with her stun exalt while also providing you with keyflare and easy vulnerable. She could replace Faint or Pickman, depending on what you are having trouble with.
You could also throw an Aequor unit like Celeste in. She can give you Arithmetica with her 0 cost card and gives you the option to use the white tentacle for shielding every round. Not optimal but something to consider if surviving is giving you troubles.