Lurker (evil flame demon thing) from Demon's Souls... what a dick... WHAT. A. DICK!! i lost so many souls trying to off this dude, learning his pattern, dodging, getting him down... then out of flipping nowhere BOOM!! 'You are Dead' *twitches*
as for SMT series, as yes, they love their 'other' bosses a lot... none tops the Demi-Fiend from Digital Devil Saga... just... OP... if you have ANY of the repels/resistances/nulls... he auto-kills your team... if you don't kill off his demons in the right order... he kills your team... if you aren't maxed level with max stats, HA! good luck, he'll flick you and you die... once he gets down to half life, if you DO NOT KILL HIM IMEDIATELY (AT HALF LIFE!!) his ***** fairy will FULLY HEAL HIM!! its repeatable!! his Arahibaki REPELS EVERYTHING!! and the ONLY way to survive is PURE GODDAMN LUCK!! REQUIRING!! that one freakin' skill that makes no damn sense 'Null Sleep' as it DOES NOT stop sleep status, IT MAKES YOU DODGE EVERYTHING WHILE ASLEEP!! to beat this guy you have to PRAY Cielo gets knocked out with sleep, have that, dodge-dodge-dodge-wake-up, hope he gets the turn, use Samarecarm to revive the others, they bring him back to life, smack-smack-smack-kill-that-damn-fairy!! continue praying and repeat!! ._.
... kill this guy, you deserve a chocolate cake...
and all it gets you is the Asura ring at the beginning of the second game which gives +10 to all stats... no sure its worth it, but its very cathartic once you kill him!! LEAST YOU KNOW THE DEMIFIEND CERTAINLY WAS A BADASS AFTER NOCTURNE!! JERK!!
and just for snark: Deus (fleshy-form) from Xenogears... not necessarily a 'hard' boss but certainly annoying... more so since fighting him i had about an 80 percent chance of the game disc freezing, having to reset, and START OVER FROM THE BEGINING OF MERVAVAH!! YOU ASS!!