like you get 100 exp and 50 points for killing your target or 1 cubelet for getting 250 ffa kills or 50 exp for playing your first soup duel for example
Sounds too easy. Achievements should require certain restrictions or more requirements for the challenger to do. Progression ones such as 250 FFA kills won't cut it unless the requirement is really high. Those should take many in-game days, if not weeks or more, to achieve.
If PvP were to have achievements, I'd say for 1v1s give a couple easy, some medium, some hard, and some very hard achievements as with the other games. For example, maybe have 1 achievement that requires the player to win a certain number of duels without taking damage or win a few times within a shorter time limit.