cbwjm
Legend
Maybe just come up with a formula to determine how to buff your bad guys, something like: For every level the PCs gain, a boss gains 20hp and a +1 to hit and damage, maybe also +1 to power DCs. The final boss will end up with an extra 120hp and +6 to hit and damage. Ideally their basic actions wouldn't really change, but they'd get more effective at using them.
You might also decide that for each level, encounters gain addition CR equal to the level of the players (just throwing out the idea, no idea if this is in any way balanced for a new level). So once they hit level 2, encounters in the next adventure will gain an additional number of monsters equal to CR 2, possibly a single CR 2 creature or maybe something like 4 orcs, depending on the original encounter. I might also add in a leader so that a group of 6 orcs at level 1 gets an orc eye of gruumsh leader when the PCs are level 2.
You might also decide that for each level, encounters gain addition CR equal to the level of the players (just throwing out the idea, no idea if this is in any way balanced for a new level). So once they hit level 2, encounters in the next adventure will gain an additional number of monsters equal to CR 2, possibly a single CR 2 creature or maybe something like 4 orcs, depending on the original encounter. I might also add in a leader so that a group of 6 orcs at level 1 gets an orc eye of gruumsh leader when the PCs are level 2.