I'm a big fan of the training concept for units, since I am really into the Special Forces units at this point I thought I would provide support for those of your struggling to level up your SF. I, at the current moment, have 8 level 20 SF (Sheila, 4 heavies, 1 lead, 2 operators) and a few soon to be, the way i go about training them is:
** Sentinel Bases
I choose to do Sentinel Bases due to the fact that they have no mines. Mainly doing level 20 since I can use rubi and do something else until the base is destroyed, this will we with any leader but don't forget to heal every 10 bases or so, so you aren't waiting around for a long repair time. Level 40 will work well but make sure you manually do it and use air where necessary. Level 20 bases gave the 2nd guy in my platoon approximately 90k xp.
** Event Bases
All event bases are very good for train SF, they have no mines and have many different layouts you can do instead of sticking with a single one that you don't like, a good way to not get bored as easily but only happens once every 3-4 weeks.
** Medal Bases
I highly recommend this method, its a bit slower but you get medals while doing them. I also recommend you do the level 30 ones, not the 60s, also have an SF with 500 range and generally atleast 8 special forces members (to take our the blitz before it deals to much damage) or repairs will be more than instantly dealt with. When it comes to the blitz you will be hit, but only when hitting them, before attacking a blitz destroy the walls in front of them, then attack, the blitz will be gone and you will be left with mere seconds of repair.
** Vercraft 50 Bases With A 500 Range Leader
This is a great way of training, for a squad with any lead but does use a sacrificial vanquisher. Use a single vanquisher to remove the 2 blitz, the orbital, and the rocket silo (if you didn't get the rocket silo its fine it wont deal enough damage to make you wait on repairs), also make sure the Vanquisher is customized. Split your squads up so the damage is not dealt to a single platoon but spread over multiple platoons. As soon as a plasma fires, or the rocket silo, take it out, it should only be able to get a maximum of 3 shots off, also take out the power plants as you walk across, this lowers the damage dealt to the platoons when you get to the opposite side. I use aggressive unless I'm hitting a plasmas or the rocket silo.
** Vercraft 50 Bases With A 450 Range Leader
The only thing that will change here compared to the 500 range leader is that it will require more vanquishers. Use 2-3 more to remove the Plasmas, once they are removed along with the 2 blitz, rocket silo, and orbital, this should literally he a walk in the park.
Goodluck Commanders, hope this helps!