That's a bit of an odd question. Can you clarify what you mean by "...gets stronger ... without spending traits..."
A Level 25 cleric should be able to walk over almost any enemy in the game and easily complete both the Arc story line and the Sewer of Horrors. Unless there's something crippling your build? So what more do you want your character to do?
There are two benefits of gaining a level:
1) Per Level Benefits such as HP, MP (which are often tide to traits and skills).
2) Getting trait and skill points to use.
So if you eliminate trait points, you're down to skill points, which can be spend in countless ways depending on your class, build, guild, and playing style. So we're back to asking clarification.
If you're looking for a class to try as a second playthrough, warrior is an obvious choice to follow-up most cleric builds. They do more damage, but won't have the healing ability of a cleric.
Sorry for being unclear. I decided to study the abilities of other class. It turns out that rogue is the only class that has growth type ability. The archery gets stronger at certain levels and stab also get stronger as well. No wonder why archery has the highest dps. It turns out warrior doesnt have that growth ability which I desire. Thanks again.
Rogues get high DPS but they lack the resilience of Warriors (as is traditional for their Glass Cannon trope). Whilst all classes get Body Development, Warriors typically have the highest Strength and Endurance stats (not needing as-much the Rogue's Agility or the Mana stats of the spellflingers) as such they get usually get significantly more hitpoints per level gained than other classes.
Keep in mind the game is designed to be completed around Level 18 to 22, so super-builds with levels in the 30's and above are rather over-powered no matter the class. So the insane DPS of a Level 100 archer / Stab Rogue Build is rather moot, other than maybe some personal revenge on trolls.
The current level cap is 100.
But there's no content for beyond Level 25 or so. The Arc and Sewer of Horrors are the current end game areas, and even those are doable between Level 18 and Level 22. So grinding for super builds is just for personal enjoyment.
Well, its not a secret that mage is getting waaaaay stronger from the beginning. At the beginning you have low hp and easy to be killed by 2-3 goblins but when your lvl grows you become way way stronger. I've played all the classes, but playing for mage was the easiest for me. I killed hard boss-enemies in solo when my companion retired in combat. I completed the arena from the 1st try for mage. I mean, you can summon giant and fat elemental, i mean its cool! At lvl 15-20 you just have to summon elemental and use staff/wand most part of your fight well maybe cast some spells occasionally.