This is a rather tough one for me.
When I saw full DF Wendy's performance against Dimaria I thought: OK, so, Gods are immune to any attacks that are not God Slayer attacks. That was proved by prime Chelia defeating her. Apparently not only did Chelia's CQC attacks hurt her, but also they managed to sort of revert her God Soul.
I consider prime Chelia's CQC attacks on par with current Erza. That is to say, they both are women, they seem strong, muscular and well-trained; with a lot of potential, and similar reserves of MP.
Now comes the tricky part:
- In the event that Dimaria has full immunity to any attacks that are not delivered by a god slayer, then Dimaria wins low difficulty.
- In the event that Dimaria has partial immunity, that is, God Slaying attacks harm her (even if their true potence is 5 in a scale out of 10), and regular attacks CAN harm her (but their true potence must be from 8 out of 10 on in order to be effective), then assuming DF Wendy's attacks are a 6-7 without the GS factor, she couldn't hurt Dimaria; but if Laxus' factor turned out to be 8-9-10 out of 10, without the GS factor he should still be able to win. In this case, Laxus would be the winner with extreme difficulty and it could go either way.
- In the event that Dimaria has no real immunity to non-GS attacks, so it's not a factor; and the reason why Wendy was helpless and unable to damage her is because her attacks are simply too weak if compared to Laxus', and I'm implying here a MASSIVE GAP (thus puting alvarez base Laxus many, many tiers above current full DF Wendy), then Laxus would win mid difficulty. I'm by no means believing that full DF Wendy is a lot of tiers below Laxus so this would be the least option I'd choose.
I choose my second reasoning giving Laxus the edge with extreme difficulty. Therefore I'm accepting that entities like Dragons, Gods or Demons CAN be 'slayed' (affected, harmed, killed) by non-dragon/god/devil slayers, but there is a "certain factor of immunity". That is to say, God Slayers can damage a God in the same way a much stronger attack without the GS factor would. So, Laxus' attacks are more powerful than Chelia's, but both would deal similar damage to a God because of this reasoning.