Well, the srious nature does not affect slugmas stats at all.
A level 5 serious Slugma with max IVs and 252 EVs in each stat has
HP 23
Attack 13
Defense 13
Special Attack 16
Special Defense 13
Speed 11
A level 5 serious Slugma with minimum IVs and no EVs has these stats.
HP 19
Attack 9
Defense 9
Special Attack 12
Special Defense 9
Speed 7
Your Slugma appears to have minimun Special stats that it is possible to get at that level.
This gives you some options.
You can
1)EV train it in special stats so that it can be more balanced.
2)Or base it around physical stats.
Slugma and magcargo both have a higher special attack than attack so you may want to use special attack on it.
If you are planning to evolve it to Magcargo, Acid Armor and Amnesia are good at giving you a chance of surving an Earthquake or a Surf. Magcargo has a VERY high Defense so it may be able to survive an Earthquake anyway.
imo Torkaol is a better option than Magcargo because it gets Curse, it has higher stats, it has a better ability, and it does not have any 4x weaknesses.
My favorite Magcargo moveset is
Acid Armor (For raising you already high defense)
Toxic (For draining away HP while you build up defense)
Flamethrower (To end the battle quickly or to defeat the opponent when you think your defense is high enough. It also gets STAB and it is Magcargo's main attacking move. It will be in most movesets that people give you)
Reflect (For the same reason as Acid Armor. You can also reflect then switch out to another Pokemon who could do with extra defense for 3-4 turns.)
Obviously, this moveset only really work with leftovers. If you have a bold nature, then this moveset will work best.
This moveset is based on building up defense. So if a Pokemon that may know surf comes out, switch to a Lanturn or something.
Some people will give you movesets like
Sunny day
Light Screen
Amnesia
Flamethrower
This agains works best with leftovers, but it is used mainly for being able to survive Surfs. Sunny day, Light screen and 3 Amnesia make Magcargo only take a 16th of the damage it would have done without any aid.
Magcargo also makes a good annoyer with a moveset like
Swagger (It can cause the opponent to damage itself a lot. Magcargo can survive the opponents augmented attack stat because of its good defense)
Attract (This is obvious...)
Body Slam (For paralysis and for Ninetales, etc.)
Flamethrower (It is Magcargo's main special attacking move)
I just though I would mention that Acid armor is available through breeding. Try getting a L.34 male Grimer with Acid Armor with a female Slugma or Magcargo.
For EVs you should try:
252 defense
129 HP
129 Special Defense
For Magcargo, base it around defense and have Flamethrower.