Giga Impact really isn't a very good idea. Double-Edge is much safer.
Slaking is like Lickilicky with Explosion.
I would much prefer Giga Impact over DB as it will assure him of many OHKO's and THKO's as I have shown with some samples of calculations. Double edge is strong but there's quite a bit of different between the two moves' raw power when it is a stab move. Also, if you have read my whole analysis (yeah sorry it was too long haha), you might notice the fact that with such bulk and power, truant is not as bad as people claim it is when such strong and defensive pokemon such as Slaking has it. With him you just have to go in, attack, kill, and get out of there (exception is with GI but unless it's a fighting move, you can take it...he can take it with ease. ALso, he would be incredible when he is paired with a dual-screener. An example is that he won't be 5HKO'd by a scarfchomp's outrage when he has a reflect on.
Anyone notice the EV Spread on the Slaking anyway?
92 HP/ 252 Atk/ 252 Def... your about 88 Evs over the max limit there
Thta was a mistake, the calculations are correct.
I mistakingly wrote the wrong EV, the correct one I have used is 92HP/252def/164atk
That doesnt commit suicide.
and it's actually incredibly bulky and will last quite a while unles you are up against...lucario,infernape and such with stab fighting move with a stats-up move (nasty plot, SD)
Slaking used Giga Impact
X takes a million damage.
X Fainted.
Opponent sends out Lucario.
Slaking must recharge.
Lucario used Swords Dance.
Lucario used Close Combat
Slaking Fainted. Your team is screwed.
That is precisely the reason.
You also forgot an important matter.
There are 5 other pokemons on your team.
If you have made your team solely dependent on one pokemon then yes you are screwed but who would do that? For the most part, the only type of team that I have seen that does that is the baton passing team with like 5 passers with one finisher and that, IMHO, is incredibly risky.
Again you could go with double edge, as I have mentioned in the analysis. It is your choice, however, I would go with GI.
Also, Lucario would have to have SD'd before OHKO'ing him (350 Atk vs 299 Def & 464 HP (120 Base Power): 198 - 234 (42.67% - 50.43).
Scenario one:
Slaking have used GI and the other pokemon is dead.
Lucario comes in and uses close combat and does not OHKO Slaking.
Slaking uses giga even for instance and Luke is dead.
Scenario two:
Slaking have used GI and he other pokie is dead.
Lucario comes in and uses SD.
Slaking uses Giga.
Of course, these are witout the fact that Slaking's life is lower than 44.83% of his max life (87.50-42.67), which is required for scenario one to have happened, which would be the only way for Lucario to kill Slaking who is already in the game.