Maingame...

Chrom

Support (any mode) - Lord -> great Lord -> Paladin or Great Knight

This is Chrom's best build for maingame. With dual strike+ he's much better off being a support bitch for F Robin, Sully, or maybe Sumia (he should never marry Olivia for maingame runs). This is because he'll be able to meaningfully contribute without requiring much EXP, keeping his resource intake at a minimum. The choice between paladin and great knight depends on who he's supporting and what bonuses they need. If he somehow reaches level 15 pally or GK (you almost never will with this build), you can either switch to the other pally/GK class or stay it in, again depending on who he supported.

Lead (lunatic) - Lord -> Cavalier -> Great knight or Paladin -> Great Lord

This is much less efficient than his support build as it requires actual resources and effort, but you sometimes will run this build. Second sealing into cavalier helps accelerate his EXP gains, and discipline and outdoor fighter are underrated skills. again, the choice between GK or Paladin depends on the support partner, but unlike his support build, once you reach level 15 in one of these classes, go to Great Lord to get aether and rightful king. You probably won't end up getting RK, but aether is okay.

Lissa

Support (any mode) - Cleric -> Sage -> Valkyrie -> Valkyrie -> etc.

She will never become durable enough to be a major fighter, so just keep her as a support bot, or maybe just leave her unpaired and have her spam rescue/physic/etc. The promotion path listed above helps maximize her mag and keeps her staff use intact (she has no reason to go for galeforce). It's difficult to build supports with Lissa because she is a useless pair up partner until she promotes to sage, and it's very difficult to have her heal her marriage partner over and over again.