Since your second Island Trial doesn't take place until you're well into Akala Island, your Pokemon's Leveling alone should help you easily deal with Totem Araquanid. However, if that isn't enough, there are still a few other things one can do to help the battle go smoothly. One of those things is having traded for Hawlucha back in the Route 2 Pokemon Center, as well as for Noibat in the Route 5 Pokemon Center. Not only are both of these Pokemon quick enough to overcome the speed boost Totem Araquanid will receive when the battle beings, but they both also brush off its attacks as though they were nothing at all. On top of this, their given Flying Type Attacks hit Araquanid quite hard.
If those two Pokemon only aren't enough, then definitely consider adding an Eletric Type Pokemon for the battle. Electric Type Attacks are also Super Effective on Araquanid, as well as all the Pokemon it might call upon for help. With Hawlucha, Noibat, and an Electric Type Pokemon, their is no way you can lose against Totem Araquanid.