I think Alegria is Mard's technique but he does need the cube to exist for it.I am not sure if alegria counts as a technique for mard geer. That was the actual base wasn't it? Momento mori is definitely dangerous if it outright ends your existence.
I am not sure if he would be hard. Take erza for example. We saw her fight good old ajir. Even when she got her powers back when she fought on her own she ended up with a somewhat ever disadvantage even after using swords which specifically took advantage of sand's weaknesses. And erza is nothing to scoff at, specially if her power increase over the timeskip was comparable to natsu's. If she grew as much as suggested then even the actual gildarts from 2 years ago would be nothing to her right now. Just look at how badly natsu beat bluenote. Once way or the other bluenote took what seems to be one of gildarts stronger moves to defeat. In turn natsu beat bluenote with a dragon roar. And in her base form erza has regularly been stronger than natsu. And then we have erza vs ajir... Ajir bested her at every turn. It took a jupiter shot and her strongest armor to barely do the trick. If natsu can defeat bluenote with ease, I can easily see the springans doing that plus some more to mard geer.
I believe Erza was outclassed when Ajeel fought seriously. But I still put EF Mard's capabilities as better than Erza's. She'd probably compare to him in attack power and maybe is better in this regard, I still got Mard ahead on speed/agility/durability/endurance even though Erza is pretty good in those stats. Basically he'd be harder to kill than someone like Erza but I do get what you're saying that the overwhelming power of a Spriggan shouldn't make it necessarily hard for them to beat Mard.
I don't equate Erza's growth by using Natsu. Erza has feats to show how much she improved, she has in fact showed the limits to her powers while the other Fairy Tail members aside from Raijinshuu so far haven't they've not been pushed into serious battle. To me she's about the level Laxus or Jura were before the time-skip which is still considerable improvement - she might be up to Gildarts level I can't tell for sure just guessing. Using Natsu's improvement is pointless because Natsu has his own set of reasons/conditions that allowed him to improve so much AND we have feats for Erza (I would understand if there weren't feats and you were using Natsu as a basis like you might do with Gajeel for example). Not everyone is going to improve the same just because Natsu has, I think Mashima has gone out of his way to portray the growth of Natsu, and has portrayed how Natsu trained so hard. Not saying anything bad about Erza here just my opinion that she's not improved as much as Natsu (through my interpretation of feats).

