There is nothing that you can do in co-op that can't also be done in solo. Other than some adjustments to enemy health and damage, having more players adds some logistical benefits that will naturally move your progress along at a faster pace than would be possible with only one pair of hands.

For example a solo player would need to be more mindful of how they manage their portals, as their is nobody else to set up the other end of a link for them. And of course you can only move as many carts/boats along at a time as you have players.

Talking in practical terms, despite having more health and dealing more damage enemies are arguably easier to defeat in co-op, as only one player really needs to dodge/block/parry while everyone else is able to commit the majority of their stamina to scoring free hits. If I recall correctly enemies get 4% more damage and 40% more health per additional player, which is more than compensated for by the frequency that co-op players will be able to force enemies to stagger and subsequently take advantage of the bonus stagger damage.

Other than the differences that having a social element might bring to the table, the changes end there. You don't have to worry about any enemies that are ridiculous to take on alone, nor worry about locking yourself into a specialisation that prevents you from enjoying everything. One player can do it all.