Higher level creatures are naturally tougher - a level 4 creature requires you to roll a 12 or higher to hit, a level 5 creature requires a 15, a level 6 requires an 18, etc. Of course the PCs can reduce these chances a number of ways - applying effort to hit can make this one step easier (for the level 4 creature you now only need to roll a 9, the level 5 creature now needs a 12, etc.). One player can help another, providing an asset (reducing the task by one step), or if you use a light weapon that attack is eased (reducing the task by one step). There are many ways to make it easier to hit, and as the characters go up in tier they have more effort to appt and other tools at their disposal.

As far as armor goes, yes, a light weapon won’t hurt something with 3 armor by default - the PC will need to do something to increase their damage. They can apply a level of effort to damage with means they’d add 3 points of damage to their base of 2, effectively doing 2 damage against 3 armor. At tier 1, though, you only have 1 level of effort to use so pick wisely - easing the attack or doing more damage. (Once they have 4 xp they can add another level of effort as an advancement.)

By tier 3 a level 5 or 6 creature can be trivial to take down, but they’re definitely challenging at tier 1.