Hard to say, as that DLC isn't even released yet.
But for theories? With the Limit Breaker skill from Ike's DLC, characters can raise their base stat caps to as high as 60, with their personal modifiers going even higher. With the right pairings, Mark can get one stat as high as 71.
Of course, that doesn't even quite answer how you'd beat hordes of enemies that are still far stronger, not to mention having better skills. But there are solutions. Being in Double with characters with the right stats, class, and supports can get you up to +10 in one stat at a time. Stacking Rainbow Cry and Love Cry gives +6 to all stats at once, which combined with another Cry for the appropriate stat gives you a total of another +10, although you'll need a character dedicated to spamming Cries to keep those bonuses up, possibly multiple of them to maintain it for the team.