As the GM, you need to see what your PCs skill values are. I look at their average OB, DB, and HP and plan the baddies accordingly. Barring the possible 1st round crit-kill, you have to make sure your NPCs aren't overpowering the party with sheer numbers or with high bonuses.
A while back, there was a thread on this board where a new GM sent two level-1 rats against two level 1 PCs and the PCs lost. What he hadn't considered is that there were 10 attacks going against 2 players. At best... 5 attacks per person, of which only 2 can be parried against, leaving very little left over in OB to even hit a rat, let alone squish 10 of them.
I'll set values where the PCs outnumber higher level foe, or a group of lower skilled NPCs with low OB, but I'll let them get position to try to get a flank bonus. Again, there's no stopping that 66 E-Crit from killing someone on the first roll.
I would be less concerned with the level of the creature and more concerned with
- its DB (can the PCs hit it with an average roll 0f 50?)
- its OB (will it crush the PCs with one hit or will they have to full parry and have 0 OB to attack with?)
- number of baddies (will they surround and get too many flank and back attacks?)