Although it may seem mainstream, try an Adamant Nature... Try Careful. Although all of mega Metagross's stats are fairly balanced, it's Sp. Defense can use some slight work. Try Careful, as it raises Sp. Defense without lowering Attack since that's what it seems like you're going for. Also, since you have that Sticky Web support, ignore the Speed stat since Mega Metagross's Speed is fair enough.
