I can tell you how I did it in Log1 for my mod, which is quite longish (~10hrs play time). I suppose with the new features in Log2 (where resuming the preview does not reset the champions) it can get a little easier, but I wouldn't rely on it 100% as you lose state anyway if you restart afaik.
Also, don't take it as a guide.. it's just a documentation of what I did
1) script a cheat function which sets the attributes to impossibly high,
but without raising the champion level. Add any quest item to the inventory. Beware of using them, but they might be useful.
2) using the cheat at 1, test every single puzzle for completion and fix them in the run. test every single secret. test every single level, every nook.
3) using the cheat at 1, do a complete playthrough outside the editor - since you are using the cheat, you aren't wasting time in combat, but you can test the plot, the puzzles, etc. Take note every dungeon level at which level and which items your champions have. Do not count items in secrets for this. Play pretending you actually need those item and switch among them.. do not rely on the fact that with 9999 strength you don't need them.
4) time to drop the cheat. remove it or disable it.
5) build a number of hidden areas in some specific zones of the dungeon. A good idea is to use a specific class of teleports so that you can destroy them in one shot when going into release. These hidden areas will contain pressure plates which will raise your champions to the specified level and which will give you the appropriate items as per notes taken at step 3.
6) in the editor, play every combat from start to finish, using those hidden areas for every time you have to stop and restart. You can skip/cheat your way out of puzzles at this stage, as long as you know that it's not a "combat puzzle". Add/move weapons and armors to balance combats etc.
7) reached the end of your dungeon alive ? time to do a complete playthrough, without those cheaty hidden areas. At this point you should have nailed most of the issues. You need a complete playthrough outside the editor. The most important things to balance at this point are food, reagents and potions. Be wise. Use roughly 2/5 of the secrets at this stage.
8) recruit some playtesters and have them play the thing
To help distributing potions, reagents, weapons, etc. the main game distribution is a fairly good reference.. in this aspect Log1 was easier, as it was linear.
Last point: play the main game. Replay it. Then again. It's an awesome reference for balance.. if you can't survive two squids at level 5, your players won't magically be able to survive squid ambush at level 1 in your mod.
