I have been pleasantly surprised with Lukas. I pulled him (Neutral IV) and figured he'd be my arena bonus but didn't expect too much. Budget SI to get Bonfire onto him and he's basically a shutout to all physical damage currently. Including greens. I wish QR was cheaper to get rank 3 of since his bulk can actually make use of the extra 10% threshold; he replaced my Nowi for this arena round so I just tossed Seal Res on him since I figured he'd wall, not kill, and I wanted to replicate Nowi's Threaten Res but he's sniped a lot of kills with 43 ATK + 22 damage Bonfires every engagement. Granted, he's basically paper when it comes to mages ... but I was running him in my Mage team of Sanaki, Julia, and Linde. Any of them can bait a magic hit in his place. It's basically a treat watching him get quaded for 0 or smack an Effie for 30 damage.
@Nowi - I've had a -SPD/+RES Nowi since forever and the amount of negative matchups (that is, she dies to) are pretty small due to the -SPD. She gains an extra set of 5 or so losses over any other negative defensive stat, most of which are greens that beat her anyway. It is basically why I went TA3/QR3 for her, though, since losing more vs greens is hardly a concern. Though if I ever have the opportunity to make her into a +ATK/-RES version, I totally would but I think she mostly gains non-meta losses.
My experience with Lukas in place of Nowi though has made for wanting to try a Lukas, Nowi, Kagero, Sanaki | Julia (since Tharja and Nino don't fucking exist) composition though. If only as a change of pace from my all mage all the time setup.