How to salvage a -atk, +res robin?

If you mean Grima, then he is still usable. Grima's Attack sits at 53 with the bane, which is still very usable. The Resistance boon gives him better magic bulk which can then be combined with Warding Breath, though you will have to exchange his Vengeful Fighter since it doesn't stack. Grima can even become a great dual-phase unit if given the previously mentioned Warding Breath, Bold Fighter, and the Quick Riposte seal.