Pokemon Sword and Shield introduces the new Dynamax mechanic, which will help shake-up the base game meta. Like Mega evolutions, Dynamax can absolutely turn the tide of a battle in your favor. However, unlike the Mega evolutions, Dynamax is significantly less over-powered as you only get the supped-up version of your pokemon for three turns. Dynamax pokemon are also not unstoppable machines, and they won't automatically one-shot you. Sure, it increases damage dealt and decreases damage taken, but the buff isn't an automatic win.
Due to the timer and the limitations on what exactly Dynamax can do, it is entirely possible to outlast your opponent's Dynamax timer and then use your own to wipe them out. Which is the kind of tactical planning that the Pokemon series excels at.
When it comes to the base combat, Pokeon Sword and Shield is much like Pokemon: Let's Go Pikachu and Eevee, in that it has significantly upped the type strength / weakness balance, so the proper combination of types can lead to some devastating losses.
The E3 demo of the game was held at the water-type gym, though, unlike previous entries in the mainline Pokemon series, the gym specialty did not automatically mean that you'd only encounter water-type pokemon in the various gym battles. However, the gym leader will absolutely focus on water-type pokemon and moves.
However, the gym included a color-gate style puzzle with coded on-off switches, which was pretty reminiscent of past games in the series which included similar puzzles along with the combat to make it so that you really, really earned that gym badge.
As always with a new entry in the mainline series, it's nice to see that Pokemon is still Pokemon even in the smallest of ways: from the gym design to the campy dialogue to the water-fire-grass type starter trio.