My strategy is a little bit different. I'm a fan of slow but sure strategy. I use lethargy and troop trample. On my troops, i used panzer samurai and frosty the bowman. I strictly use my diamonds to upgrade sword lvl up to lvl 10. bow lvl 5. Frosty lvl 4 and panzer lvl 5. Hero lvl 9, sacred gate lvl 3. Lethargy lvl 5 and troops trample lvl 1.
Panzer, once you get into lvl 5, is a hard wall to breach in lvl before 50, but is still a weakling to the lvl 50 boss. Especially when the spider zombie comes around. So my strategy is:
Early stage: use your hero to fend off the enemy's army. Meanwhile, upgrade your leadership to the max without even summon any unit. Use lethargy and troop trample to defend your position and kill as many small armies as you can, and keep the boss busy pounding on you while you evade it, so it won't step much further. Don't worry, before you realized it, you'll be already kill a large portion of the earlier troops because the game spawns a weakling troops in the early on stage 50th.
Middle stage: by the time you max out the leadership and able to summon your first panzer samurai, the boss will most likely already passed the middle bridge. Usually he will be around the warding bell position. Now is the time to be smart and fend off enemies attack from behind the panzer samurai's line. Usually a panzer unit will die in about 5-7 seconds. That's why you really need all the slow in the world. Keep summoning the panzer, but alternate between the panzer and the frostie summoning in the way: 3 panzer, 1 frostie. You need to ALWAYS stay at the front line, just in case the panzer supply is short. If the small fast round enemies get past you, let them be. Just kill them using the troops ability. Or if your gate is not that strong enough, strike lethargy when those small enemies come, and kill them fast. Do this until all the enemies that accompany the boss are wiped out, leaving the boss alone. Don't worry, it may seems hard at first, but once your frostie population is enough, you will see a never ending lethargy. :) oh yeah, whenever you see those enemy's bowmans behind the boss, hit lethargy, and quickly wipe them out. They can be a serious threat to your panzer if accumulated.
Late stage: when the smaller enemies had been wiped out, this is where the easy parts comes in. Just keep summoning the panzer, as the boss will never be able to wipe your panzer with his very-slow attack, thanks to the frosties. But your real damage will be coming from the troops trample attack. So just keep bashing the lethargy-troops ability, and panzer summoning, until he's dead.
I didn't use any sushis, tea pots, or charms with this strategy. :) but yeah, i failed a lot before "getting it right", because this strategy also needs mastering of the hit-and-run hero strategy. The good thing about this strategy is that you don't need to be distracted by keep mashing on the farmer / assassin button. Therefore, you can concentrate on maneuvering your hero, and keep engaged.