Normal in Serious Sam is relatively brutal on your own - in co-op you have respawns, in SP only quick-/auto-saves. And with enemies teleporting and swarms and large hordes, the first time will kill you plenty either way.
Serious Sam is not a casual stroll through some thugs with the occasional cheap boss. It's always a murderous rampage.
Also, in defiance of the modern shooter plague I'm playing through it aiming for the Old School achievement - in which you are not allowed to manually reload your gun, sprint, or aim down the sights. I love Croteam so much for including that achievement, and it's an extra excuse for playing "only" on normal. The assault rifle zoom and the sprint can probably save you a few hitpoints every now and again.
