If you don't mind cheeses, any nation in the vicinity can easily kill Rome 3 years in. (2 for fabricate claim, 1 for the war)
Day 1 Fabricate Claim on Latium, get military access all the way to his north, hire a stack of merc, and march the merc stack and the levy over.
Turn off Surplus in Capital and low Maintenance if you need to keep your income in the positive. (I also had to disband my navy while playing Massalia)
As soon as the claim is finished, DoW, march your troops over, Assault, and sit back waiting for ticking WS and 'months at war' modifier. May want to Integrate Roman Pops while you're waiting.
His troops will be busy sieging his southern neighbor, so you have plenty of time to swoop in, Assault, and retreat.
As soon as he annexes his southern neighbor, you can demand Latium without "-20 taking significant land" modifier. If you still need more warscore, wait till his doom stacks are sieging the forts, and snipe a couple more forts with Assault or take some of his unprotected Provinces.
Take Latium, and annex him on the next war.
If you can manage to ally Etruscan, you can even fight toe to toe and completely neuter Rome in the first war.
If you don't want to cheese it, just ignore the mission tree, and go ham with fabricate claim to push north until you receive the generic conquest mission.
Annex -> Imprison important character -> Sell to Slavery for Tyranny and 100~200 gold -> Hire merc -> DoW -> Assault forts down-> Dismiss mercs, and repeat.
As soon as you're regional power, hire the 2 biggest merc stacks you can find, turn on Appeasing Stance, ally any small Hellenistic culture nations, gift, turn them into Feudatory.