No, we don't like the option of finding one easy to level critter and that being ridden to 21 in no time at all. Killing critters needs more critter classes and better base xp for them.
One is a technical issue and one will hopefully be done before too long.
Why don't you make diminishing exp rather "player lvl vs critter lvl" based than decided by number of kills? Its nonsense not be be able to level up on critter that you have killed more than 10 times. Imho it should work like this:
Leveling up will be the most effective when player kills critters that have the same or similar level as him. When he do so he will get 100% exp no matter how many critters he kills. But when he will kill critters that are significantly below/above his level he will get lesser exp (for above its to eliminate higher lvl player boosting lower lvl friend). So greater the difference between players and creeps level is, smaler portion of exp player gets from such creeps.
This will require many changes in current creep system. Creeps should be level based meaning we could encounter lvl 1 rats but also lvl 6 rats. Distinguished from each other by their model size, contour color, skin color, additional graphic elements (possible more or less only with 3d models). This would mean we won't have only 1 kind of creep but also additional sub-kinds of the same creep leaving players wider choice of their prey. Also something like tough creeps could be implemented. It should be something like "elite" version of their kind so something like deathclaw mother/extra strong molerat etc. They should reward player with more exp and in some cases even some items near their nests or something. Also some easter egg creeps could get implemented to special encounters (Many-as-one rat from PST, Ian from original game etc...). But my point is to make leveling fun and not so time consuming for PvP chars.
With this system it could look somehow like this: You create your char, you will have to get yourself a tent or base asap, then it will require some ammo/weapon to get in busyness. When you have some you could start killing creeps for exp. At the beginning pretty much anything will do the trick since low lvl creeps are common. But at higher lvls you would have to search for creeps that are near your level = you won't have easy exp but income will be stable. Also if you decide to go killing some centaurs/floaters/aliens/fire geckos right from the beginning then you will fail hard. Those creeps would give you only small portion of base EXP if you are like 3+ levels under their level. So your EXP income could be pretty much balanced since rate "exp gained from creeps : exp needed for next lvl" would be easy to moderate. First few levels would be easy to get, middle would require some harder creeps and better exp reward from then while low lvl creeps would give you again only small % of base exp and when you reach certain lvl it would be effective to take down some hardcore mobs like deathclaws/floaters/centaurs. And since ammo/weapon requirements would grow with each level it seems to me like the best way how to make lvling system. No easy exp yet stable income of exp, also requirements for killing creeps grow with every new level (since higher lvl = need to kill harder mobs that have more hp = more ammo used/need for better weaponry, more players to kill them etc).
And something so elemental as sharing exp between party members would help drastically.
