He got them relegated out of sheer stubbornness in regards to his play style and his complete inability to organise a defence. If he hadn't stuck to his philosophy of "pretty" football, he could have easily kept Wigan up. There's a reason why the so-called survival specialists like Allardyce and Pulis play direct, "ugly" football, because when you're in the thick of it all that matters is getting points, not how you get them. If Martinez had spent more time focusing on organising his defence properly at Wigan, and less on the technical side, they would've had a much better chance of survival, just like when Jewell and Bruce kept them up.
Now the same is happening at Everton, too much focus on the attacking and technical side, whilst their defence gets worse game by game seemingly. John Stones is almost Martinez football incarnate. Yeah he looks good on the ball and is quite good technically, but he makes some horrendous mistakes all because he's fannying about trying to look skillful. The amount of times I've watched Everton highlights this season and they've given away a goal/chance because Stones is dribbling in his own box or trying to play fancy passes with opposition players in front of him is baffling, and yet it still happens, a proper manager would've told him that sometimes you just need to word removed the **** as far away from goal as possible.