First diplo win

All the usual options to get a war started apply: the best option is obviously to 'persuade' your designated Rogue to DoW you (e.g. by refusing to pay them tribute, or by appearing Weak) -- that way, they've given you a cassus belli (and WH). Or you can wait for them to attack a neighbour, then sign an MA with that neighbour. Or if your Rogue just won't play ball, then you can wait until any active per-turn deals with them have passed the 20T-mark, then issue the DoW yourself (just cancel the PT). In the late game, MPPs are also a possibility, provided that the Rogue's neighbours aren't likely to DoW each other by 'mistake'. Or you can ensure that any MPPs you do sign are then activated the way you want them to be, e.g. by signing MPPs with the neighbours, then immediately DoWing the Rogue, and leaving a weak bait-unit (e.g. a Slave, or a Curragh/Galley) within reach of their borders. If you can't be 99.9% sure of your allies though, then it's better not to sign MPPs until just before a UN-election is due (each active MPP+RoP will pretty much guarantee you a vote).

Once I'm at war, I prefer to pay gold-per-turn for MAs, because the AICivs almost always sign PTs before the 20T is over; if they're the ones who break the MA-deal, I think I then don't take the rep-hit. Paying them everything upfront is less rep-risky, but obviously costs more in the short-term, and gives them no incentive to keep the war going longer than they 'want' to. But if I have some useful (defensive) mil-tech such as Feudalism or Gunpowder to bribe the Rogue's neighbours with, so that they take less damage in the process, then I might throw that into the MA too (or sell it at a discount in a separate deal).

If you have spare Luxes available, then it may be worth gifting them to your allies to reduce their citizens' WW. Gifting not only makes you look good, but (I think) if the AI is paying nothing for them, then getting the export cancelled through no fault of yours shouldn't give you a rep-hit. So again, try to avoid including a Lux-export as part of the MA-deal (if possible), especially if your ally is one of the smaller fry: if they lose their (usually one-and-only!) Harbour-town to the Rogue, not only will that cancel the Lux-export and hence the MA, but you will be considered the deal-breaker. (Maintaining your own Harbour-town(s) on your allies' Continent can protect against that possibility to some extent -- but only if you can be sure of holding it/them).