Ajeel can take rounds 1,2,3 Easy difficulty. Round 5 would probably be a bit closer to mid diff if we're talking current Jura. Ajeel will have to get a bit serious for that one I think. He should also be able to take round 6 with high difficulty. I believe he'd need sand world for that and if he's using sand world, it does mean he's very serious. With the GoI teaming up on him, he'd have a fair bit of trouble. Even though Serena initially incapacitated them, they managed to quickly counter and turn the tables, and probably would've won if Serena didn't have multiple DS forms. Even Jacob felt the need to step in at that point. Considering that, Ajeel would have a bit of difficulty completely defeating them.
For round 4, honestly can't say if he'd win. Natsu did defeat the god but he did it with a FDK spell. Sure we're not exactly sure of the details but Natsu's initial attack didn't even make a scratch. Ikusatsunagi also packs a great deal of power and his ground splitting attack with a single swing is actually one of the highest AoE attacks we've seen. Considering his size and range of attacks, I can actually see Ikusatsunagi being able to disperse sand world by using a sword swing to blow the attack away. Well, whether or not if that's possible, I think Ajeel will lose this. Probably mid diff for Ikusatsunagi