Best game in the series so far. You improved a lot from the last.
TIPS
1) Abuse Shadow's absorb ability. No seriously, you have no idea how helpful it is to recover health without needing SP
2) For the first 2 turns (from a team perspective-as in after Sonic and Shadow's turns combined) have Sonic and Shadow just attack Seelkadoom, and I mean attack as in press the attack button. Those small changes will help you a lot in the long run.
3) For the third turn (team perspective) do double blast just to get your headstart on the damage.
4) From turn 4 forward there you literally can just abuse Sonic's holy drops, Shadow's absorb ability, and the heal skill when you get down to really low-half health (I would say at typically 40%-60% health use the drops. Absorb you can basically use every turn). The exception of not using these is if OL bar is full or if your health is low when it is).
5) Wait until both parties have their OL bar up, the full combo should do just about 20K damage-which for reference Seelkadoom has 80K health I believe. Up arrow, down arrow, left arrow, right arrow, and enter are the combo keys.
6) Important: Shadow's Royal Shield skill whilst useful IS NOT NECESSARY to win. Not to mention 100SP is obviously an expensive price.
7) If Sonic dies somehow-like if you just wanna have fun with it so you used sacrificer to kill him. At least make sure Sonic uses all of his SP. Keep abusing Shadow's absorb ability. At this point in time you may as well heal when you get low as well with Shadow.
8) Not if Shadow dies, this is where it gets interesting. I barely won with Sonic literally using most of his SP to heal and I think I finished Seelkadoom off with a regular attack. MidNightMaren obviously intended Shadow to be the one living since Shadow's abilities protect you more. (Royal Shield negating all damage and absorb taking no SP for a fair amount of health. In contrast Sonic's has to rely on heal which is 25 SP whilst holy drops which heals both parties is 20SP...not a big difference in the risk but a big one in reward).