Tips on best places to level please.

I am around level 52-78 (My second party is pretty weak.) Where would be the best place to level for my second party and my stronger one (lvl 73-78)

for both id say the henne mines though your weaker ones might struggle a bit cause those abbysteels are hard lil feckers though you could have 2 of the stronger characters and one of the weeker ones fightin at each time

failing that take your weaker dudes to the nabreus deadlands

Cerobi Steppe - for starters

Nebreus Deadlands (overlooking eternity) - for good chains and exp.

Henne mines (east area found on the way to get Zodiak esper) - for advanced

GameFAQs: Final Fantasy XII (PS2) Negalmuur Auto-Leveling Guide by xRestless

Use this till you hit 90ish, then Abysteels, this takes too long 90+.

The Abysteel trick works wonders Level 70+ (Pithead Junction C. Sleepga -> Curaja [they're undead])

Before that, Nabreus Deadlands, Overlooking Eternity. Go nuts on Dead Bones. You should get about 20-30 chained before you need to zone out and come back. You'll make a lot of money selling the loot they drop too.