On entering Fontaine, you are immediately recognized and greeted by its Warlord, Motochika. He asks your name, seeming surprised that you came after him rather than waiting for him to attack Aurora. This clearly isn’t too offputting to him, though, as he keeps his cool quite easily and just agrees to battle you in Fontaine.
Once you hit the battlefield, it immediately becomes apparent where the real difficulty of this fight comes in. The battlefield is cut into two large sections and one much smaller one by a series of channels leading downhill from the right side of the map to the left. While the hills themselves are surmountable and the channels shallow enough for most Pokemon to jump down into them and then back out, they are flooded with water, rendering them completely impassable to anything that is neither a water-type Pokemon nor airborne- this is what makes that chance of having a Carnivine especially valuable.
Compounding the problem, your attackers- with the exception of some of the Fire-types you picked up in Ignis- are pretty exclusively restricted to melee-range attacks, while every last member of Motochika’s forces have attacks that reach two squares. This means that while they can strike at you from across these channels, you cannot return the favor with anything that’s likely to last for the entire exchange of blows.
Fortunately, there’s a way to handle this- a pair of floodgates, each blocking off one ‘layer’ of the channel from the section immediately downhill of it. It’s the use of these gates, which toggle between open and closed each time a Pokemon ends its turn on one of the adjacent buttons, that can save you, and it is for this that you may not be bothered too much by bringing a Fire-type Pokemon along. Be cautious though- make sure that if you have a fire Pokemon with you and use it to close floodgates, you do that in the same turn as you move more forces across the channel. If you don’t you’re likely to see the Pokemon wiped out and the gate opened back up during Motochika’s turn without you actually getting anywhere.
What’s worse, if a floodgate opens while a Pokemon is in the affected area of channel, the flow of water will go so far as to knock the Pokemon clean out of the channel. While this may leave your forces across the water from where they had been before, you have no control over where they land, and the water damages them in the process. Being water, it is especially damaging when it does this to a fire-type Pokemon and not terribly damaging at all to Water-types, so be wary- Motochika may not mind at all if he finds himself wanting to open a gate that will flood one of his Warriors’ Pokemon out of a channel.