Cursed with slow start regigigas was doomed to the NU teir.

depite Boosting a base AT of 160
base HP, DF, SD of 110 (some thing every tank is jealous of)
and a respectable SP of 100

Slow start takes most of this away, halving SP and AT
its base SP is equivalent to a base 25 SP, Base AT is equivalent to a 55 AT

an Adamant Max AT Regigias with out slow start hits 460 AT with no items devastating almost everything

In Gen V the game introduced the move Simple beam which changes the targets ability to simple (except traunt and mulitype) simple doubles all stat boosts/drops
with the move rock polish and slow start regigigas can have its speed boosted 4 stages in one turn
raising adamant Max SP regigigas to a speed of 897
Thats faster than a timid/Jolly Max SP Deoxys Attack form that used agility! (876)
Genosekuto can use the move simple beam. A good choice for a special sweeper ally

Xenon3120 made a video about slaking and desukan if you attack a pokemon with the mummy ability with slaking would lose the traunt ability and gain the mummy ability, however if you switch out do you lose the mummy ability?

I really like Regigigas. Especially that kind of Regigigas. But how do you get to the Pokemon smileys?

I am pretty sure that if you do switch you lose the mummy ability. Also, I don't see this being effective in anything outside of double battles, or in the new generation triple battles. People have been trying to figure out a way around Regi's crippling ability for ages, and with 80 base stat SpAtk it is hard to call it a special sweeper at all especially with it's horrid special movepool. It has 4 good special moves that could be used, but no Calm Mind or Nasty Plot options.

If it were me, I would make a fun stat boosting and screening BP team, and then send out Regi to take a few hits and then have his way with the team. Either way, nearly 4 years later there is a reason that Regi is still in the NU tier. I am sure we will all just keep dreaming about Regigigas being a force, but I don't see his usage in OU being realistic. But that is just my opinion.

Who said anything about him being a special sweeper?
and of course it only works for double battles

besides I just wanted to point out
that there is a alternative to worry seed and Skill swap
no one used skill swap because it crippled the user until you switched out
Worry seed didn't work because for the most part the pokemon that wielded it were fragile grass types

there is also the move Make friends which replaces the targets ability with the users
Aianto learns this move, and gets the ability Hustle which would boost Regigigas AT to 690
BTW Make friends doesn't work on Traunt.