Impale can also be targeted on empty tiles, which is useful for getting off an attack while trying to close the gap but not quite managing to reach enemy lines, or when trying to catch a fleeing ranged unit.

All of these things add together, so with all the stuff you mentioned, you'd have 85% of your normal experience gain, not 125% and not 25%. (100 + 15 + 25 + 10 + 10 + 25 - 100)/100 = 85%.

I also disagree that Potential itself is useful if you have all that other stuff. Potential on its own will only give you an extra level by the time you'd normally be level 20 (at which point the Potential champion is even with the non-Potential champion in terms of trait count); with all this other stuff, assuming you have it all by level 3 (which is when you can start benefiting from Potential), the champion with this stuff and Potential will not match the hero with this stuff and without Potential in terms of real traits until a champion with none of this stuff would have reached level 13 (at which point your Potential + equipment champion is level 16 with ~100XP earned towards level 17, and the equipment-only champion is level 15 with ~100XP left until level 16); moreover, these two hypothetical champions will more or less stop leveling up within a few levels unless you have significant numbers of powerful monster stacks for them to go after. Note that this is the point at which the champions become equal in terms of real traits, not the point at which the Potential champion gets a full level or more ahead in terms of real traits. (And yes, prior to this point the Potential champion will be fractions of a level ahead of the non-Potential champion, but this difference is essentially equal to [current level]/14 in the case of the Potential + equipment champion, or [current level]/20 in the case of the Potential-only champion - which isn't that much of an advantage when you consider that a Potential champion at level A is roughly equal in power to a non-Potential champion at level A - 1, because the Potential champion blew one of his/her trait slots on Potential instead of something that provides actual power, and the non-trait bonuses from leveling are miniscule compared to the trait-related bonuses; I will admit that there are exceptions at lower levels when having an extra +2 attack from some weapon that gives attack per level or having +2 health matters a little, or at middle levels for some of the high-end damage spells, but a Mage is far better off taking Knowledge than Potential or even Potential with Knowledge - after all, Knowledge on its own will actually break even at a level you might reasonably expect to reach in normal gameplay.)

If the goal is to get the champions to the highest level you can bring them to, then yes, Potential will work for that, and it may even be worthwhile on a champion you're going to concentrate on to turn into an abnormally high level, but if you're just concentrating on making the champion as strong as possible for normal gameplay Potential isn't worthwhile because it's just never going to compare to just about any of the other traits available in terms of the power resulting from taking the trait.

Better is an army lead by a Cautious Sovereign/Champion (or one with an Escape Scroll), since you don't need to sacrifice more units than fall in the time it takes you to finish as much as you care to - after all, you can run away. As they say, he who fights and runs away lives to fight another day, but he who is in battle slain cay never rise to fight again.

The problem with this, of course, is that usually you want your sovereign leading the big armies that beat everything they come across, rather than the weaker armies that bleed the enemy before a bigger army comes by to swat them, and your sovereign is the only unit you have access to that you can guarantee is Cautious, and Escape Scroll access is unreliable. Still, a Cautious Sovereign leading a fast army can do a series of hit and run attacks, assuming that the army has enough movement to close, attack, and run far enough away in one turn to keep this cycle of attacks going.

I get the feeling he's referring to Mage champions rather than Mage units, since he said "spam its spells upon the juggernaut," which makes crystal availability less of an issue (though that depends to some extent on what the Mage gets as an escort, if anything). Still, if you have an "army" of decent Mage champions, I don't really see why you wouldn't rather sprinkle them in as commanders of real armies instead of making a big stack of champions that blow through lots of mana to kill one or two units so that the main army or armies have a slightly easier time taking cities and crushing armies.

This is true, especially if you use Finesse, since you can almost guarantee that a spear unit will have higher initiative than anything which uses axes or clubs, and may have better initiative than other spear units, and sometimes sword units, depending on trait picks and training center quality (specifically, essence and Aura of Grace). Other traits offering offensive bonuses also help, of course, but Finesse is the one trait that sword and spear units are far more suited to use than axe and club units are, assuming that you're using the same armor on your units that your enemies are (or that your armor is the one with the lesser initiative penalty - e.g. Chainmail units up against Light Plate units).