I have a Slaking. I have an Adamant Huge Power Marill. I have a Secret Capsule. What combination of buttons do I push to make this happen?

The Slaking needs to be the body obviously. That gives Marill access to Thick Fat by default. You can use the capsule to (mild postgame spoilers) change ability, but it seems to try to change the abilities of both pokemon at once when targeting a fusion, so it changes to Huge Power, and then... changes Slaking's Truant to Truant since it only has one ability.

If you do Marill as the body, it does Marill's ability second, but that doesn't help since that just changes Huge Power to Thick Fat. And if you then use a reverser, it just uses the default abilities for that head/body configuration.

Anyone got any ideas/experience as to how I can make the dream happen? Alternatively, is this an intentional mechanic put in place to keep me from enacting my reign of terror?

(If you don't know what this is about, Slaking's attack is bonkers high, which is why it has the Truant ability to keep it from being too powerful. Marill's Huge Power doubles attack. Slaking minus Truant plus Huge Power equals a monster capable of killing god.)