One-Shot Bull-rushing Boss

[Madara]

Running a one-shot tomorrow, likely with fairly inexperienced players. I want to have a boss who's essentially Vi from League of Legends, so charging and bull-rushing is ideal. The game will have premade PC's who should be under ECL 3 for ease of use. So what class should I go with for a low-level enemy who bull-rushes?

[Nettlekid]

Honestly, Fighter. Dungeoncrasher Fighter (from Dungeonscape) is all about smashing people into walls and doing lots of damage. The first ACF is level 2 and the next is level 6, so you could go either high or low. I might suggest Barbarian 1/Fighter 2 on a Warforged Charger for a low-level smasher.

[Silva Stormrage]

Dungeon Crasher + the Knockback feat is a deadly combo

If you want to go massive optimization (Might not due to pre made PC's) gestalt fighter with anthropomorphic whale would be freaking devastating.

Otherwise Goliath would be a good option for getting Knockback, or half minotaur but might want to avoid that one to avoid a tpk.

[Seclora]

Gonna have to second[Edit: third it would seem] Dungeoncrash Fighter. But if charging/bull rush is your combat style, I'd take a Dwarf(Stability) or Half-Giant(Powerful Build) over a Warforged Charger(CR 5, higher if you trade HD for levels).

If its a one-shot, I'd also build a bit of a custom arena; spikes on the walls, narrow walkways over a pit of lava, the sort of place someone who's main tactic was to push people into things would want to fight in, in an ideal circumstance. Or maybe just narrow walkways with a 10ft pit filled with fog, since you mention that the players aren't terribly experienced.