I really don't like the term "pseudo legendary" anyways. I understand that it has to be Pokemon that are not considered legendary, but the requirements seem a bit arbitrary to me, particularly the requirements that it must have 1.25 million EXP at level 100, and a base stat total of exactly 600, in addition to the thing about having three evolutionary stages.
However, the thing that cripples Slaking is Truant, and that alone is enough to cripple him, which means you need another Pokemon that is able to remove or change that ability. Worry seed, Entrainment (the opposite of Role Play, and Simple Beam will not work for this, so you will need to use Skill Swap, Gastro Acid, or the ability, Mummy, to realize this beast's full potential, which pretty much limits its usefulness to double/triple battles, unless you were to able to give it pursuit, and use it on someone who leads with a Confagruis/Yamask. No wonder he wasn't given a mega evolution, If you were somehow able to get Huge Power or Pure Power on a slaking, he would be a force to be reckoned with, with a possible max attack of 2,760, (+6 attack, and huge/pure power boost).