At high / top level, the most-used unit (both on offense and defense) is currently the boosted mummy together with the knight boost.
Boosted mummy spawns almost a dozen of knights in place when it dies, and with knight boost that means a dozen of new boosted knights spawning wherever you and your mummy currently are. It also means you can use a hostile skull tower’s bomb volley for killing some mummies and spawning knights instead of having to kick back all the bombs to save your previous knights…
Thus, with boosted mummies you get only slightly less knights per morale than with directly spawning knights, but you can spawn knights anywhere, thus also to the last few seconds at a doom gate (compared to like 50s before end of raid for regular knight spawning, as they have to run through the whole base first), and your knights are on average less exposed to hostile ranged troops.
Of course, getting used to the boosted mummy takes a while, because you need to practice the proper timing of spawning and spells.
If you spawn a mummy and then cast a shield spell… well, the shield spell is wasted (only 1 unit in range), and the mummy lives longer, thus it takes longer before you get the boosted knights and you lose valuable time. On the other hand, a high lvl skull tower might kill a mummy and also damage the newly spawned knights unless you kick back the last few bombs, so launching a shield spell as soon as the knights spawn (about 1s after the mummy dies) is a good idea.
Spawning too many mummies can clutter up the path, causing your other units (and knights) to get stuck behind, and as mummies are way slower at destroying barricades than the knights are, you will lose time.
Losing your knights in places without any concentrated hostile fire makes you have alive mummies for quite a while before they die, potentially costing you time, so you have to think ahead of where to sacrifice / let go of your old knights and use a spawned mummy for getting additional / new knights, and where to care more about protecting your knights at all cost (even if it requires casting a shield spell on a mixed group of knights and mummies and kicking back all skull or boosted bomb tower bombs in a certain area).
Generally, mummies can also be a great way of “transporting a compressed army” to avoid hostile area damage (only 1 mummie = 1x damage, compared to 10 knights = 10x damage), and can make great meat shields for your king, as they even create a “second meat shield” (knights) when they die. But of course, taking advantage of this requires good thinking and timing again.
On defense, using boosted mummies (again, together with the knight boost) is relatively easy: Just add mummies to your waves!
(Some advanced details of course exist there, but it’s hard/impossible(?) to do it completely wrong, and finetuning always depends on the individual base layout, as for any other troop type as well.)