I can see Diggersby being a Hit-and-Run Spiker with Spikes and U-Turn, possibly with preferred Sticky Web support somewhere in the mix; Sticky Web in general is good for any Diggersby, but it may actually enjoy having a fast U-Turn to escape with since its typing gives it some decent synergy with other Pokemon, but at the same time leaves it vulnerable to a number of faster threats. Probably something like:
Diggersby @ Leftovers
Trait: Huge Power
EVs: 252 Atk / 232 Spe / 24 HP
Adamant Nature
- Earthquake
- Quick Attack/Stone Edge/Return
- U-Turn
- Spikes
With these EVs, you're able to outpace a -1 Greninja, probably the fastest Grounded threat which you're capable of OHKOing with Earthquake and an Adamant nature. The ideal behind these EVs is mainly that you have Sticky Web up or at least have a reliable setter of it through the match, so that most Grounded Pokemon get hit as hard as possible. Stone Edge is optimal for having better coverage alongside Earthquake, but Return lets you get both your STABs at full power at the cost of some useful priority.
You could also use a spread of 110 HP / 252 Atk / 146 Spe Jolly to invest more in bulk, but your power level will be ramped down a bit, and you'll just barely fall short of OHKOing Greninja from full HP if you don't run Return.
That's probably the best I can come up with for a Spikes set, it'd be a good user of it on Bulky Offense while still hitting pretty hard, at least.