There are many different types of Pokemon for example your Piplup is a
water type and when it uses water type moves it will do extra damage
(because the move is the same type as itself) Piplup can easily take down
fire, ground and rock type Pokemon.
Ekans is a poison type Pokemon and is weak to ground type moves once Piplup
is poisoned it will tell you on it’s menu/battle screen.

Only tip I can give is don’t fight high leveled Pokemon unless you have the
right types to take it down or the right amount of Pokeballs etc to catch
it.

Some tips for you!
So, you know about EVs and IVs. That’s great! Now, shrug it off.

Wait, what?
Yeah. As you can see,

>Pokemon distribution in Pixelmon is vastly diverse even in one biome, EV
training is going to be extremely tedious because you have to search for
some specific Pokemon to KO, which is not the most fun thing in the world.
Not to mention your Piplup is only level 5-10, and some Pokemon needed to
EV train is going to be on high level. This can be alleviated if you have
Power Items, but for now, neglect it completely.

>EVs affect your stats, but the effects is only going to be significant on
high levels (on level 100, for example, 4 EVs=1 stat point). Even then,
your Piplup’s IVs are randomly generated, which means most likely it won’t
be anywhere near perfect (perfect number is 31, by the way. You don’t
always want perfect IVs in case you need coverage in the form of Hidden
Power, though). Since breeding is nowhere near being added in Pixelmon, you
have to catch a copious number of the SAME Pokemon in order to get a
Pokemon with even one perfect IV.

>There are several ways to build the set of your Pokemon. Your Piplup, for
example, once evolved into its final form (Empoleon) is good for Special
Walling or Special Attacking. Since you have Mild, you’s better off making
it to a Special Attacker. But, Empoleon needs team support quite badly,
which then limits your options on teambuilding. For now, enjoy Pokemon as
it is with some fun ways to do them (putting Hyper Beam on Gyarados, for
example. Yes, Hyper Beam is a bad move on Gyarados, but who doesn’t want to
see a laser-shooting serpent?) Competitive Pokemon battling is a different
story, especially considering not every Pokemon is added yet, causing the
metagame, if it has one, to be insanely unbalanced.

>Learn some basic game mechanic first. Pokemon Types, Pokemon Evolutions
(Piplup evolves at level 16 to Prinplup, and evolves again at level 36 to
Empoleon, by the way. Some Pokemon have other ways to evolve, though), Move
types and effects, Critical Hits, Type effectiveness (learn the Generation
5 Type chart, not the Generation 6 one), STAB (Same Type Attack Bonus, eg
Bubble is stronger if used by a Water-Type than if it’s used by
Normal-Type), Status Conditions (Primary: Freeze, Sleep, Paralysis, Poison,
Toxic Poison, Burn, Secondary: Confusion, Attracted, Leech Seed, Flinch,
Trapped, etc), Pokemon Stats (HP, Attack, Defence, Special Attack, Special
Defence, and Speed), Pokeball effectiveness (I recommend making Dusk Balls,
they increase your catch rate in the night by 4 times as opposed to Ultra
Ball which is 2 times [anytime, though]) are fundamental, even on casual
players who don’t care about EVs. You can learn this stuff at Serebii and
Bulbapedia. Smogon is always there if you want to know some more complex
stuff, though.

Wow, that sure was verbose. But don’t let that daunt you; Pokemon is fun as
it is!
TL;DR: Leave EVs and IVs at bay (at least for now), learn basic mechanics,
have fun!