PSDuckie, at the beginning of your adventure you will be given OmNom the male Mudkip. OmNom is small and cute but whises to become big and strong, a real boss of the swamp! He's just too little and needs to grow, he must fight and eat to make his whis come true. So as a Mudkip he must keep a berry, and every time you find a new berry he wants to taste it (different foods give different nutrients, he must follow a variet diet), also he needs to take all the vitamins you find if they have some effect on him. Remember that he's still young and frail so he must switch out whenever the opponent weights more than 45lbs. Sooner or later he will learns the skill of Taking Down his opponents with his strength: after learning Take Down and soloing Wattson (not necessarily in that order) he can grow and evolve to Marshtomp. Remember that he's just a little Mudkip now, and must switch out every time the enemy weights 50lbs or more.
Now he's quite strong but still not enough. He still keeps eating all the berries he can, and wants to know at least 3 physical moves: one is Take Down and one must be Strength as you get it. Now he only fears enemy weighting more than 150lbs, but he must solo any trained pokémon between 100 and 150lbs, or he will never be able to defeat stronger opponents! In order to evolve again he must be brave and able to get over this limitation: he will need to solo Norman scoring the KO on both Slakings with Take Down, and score both KO on Tate and Liza's Solrock and Lunatone without using Water-type moves.
He finally becomes a great Swampert! He must learn Earthquake (from level up if possible, or by TM), and now he doesn't need berries anymore: rename him BOSS asap (the weight restriction is also gone). Unfortunately, he realizes that nobody can teach him Double-Edge...so replace Take Down with Frustration. He will also solo all Water-type pokémon of E4 and Steven's Skarmory to soothe his disappointment.
(here's a list to see pokémon's weight
, if you find boring the weight restriction I can change or lift it)