What disc are you on? Doomtrain & Cactuar can only be gotten on Disc 3+4
As for Tonberry King you have to kill 18-22 tonberries in the cetra ruins. before being able to fight him. I think he levels up as you do (like most bosses). Good junctions and a strong party should do it.
It's a good idea to fight him with a level 30+ party, as you can get full-life from him, which is a pretty powerful spell added to your arsenal.

As for doomtrain, you need to aqquire some items on disc 3 here they are:
1 Soloman's ring (you get this at tears point)
6 Steel pipes (Mug some Wendigos)
6 Malboro tentacles (found on disc 3 onwards)
6 Remedy+ (Use Alexander's med-level ability)

And finally for Cactuar! Use water based junctions or summon Leviathan. Cast meltdown. Use limit breaks.