hm.. 8 is quite an amount...... but sure u will come out with two quite strong betas.Fierce betas doesnt mean it will be strong.Some betas tends to look fierce but once it is place into fights, it will open it gill and swim away.
Anyways, i do have a way to train beta,but it is quite cruel.If any of your betas werent strong enough... it might die.One thing.... dont choose those fancyful betas(half moon or whatever moon tail betas).Choose those that have shorter tails.

+Keep 8 small jar(make sure it is covered with black so that no light can go in.)
+Make sure the jars are ventilated.(u can either use a black paper and cover over the opening and poke a few holes on that paper so air can go in).
+Put those 8 jars at some place where not much light can reach it.
+Of course do your maintanance as usual(Feeding,water cycle and dechlorinate your water as usual)
*Regarding the food,make sure not frozen food or flakes.Feed it with live bloodworms or tubifex worms.*
+Juz make sure no more neccessary approaching for two weeks.(Like peek at your beta or look at it).
+Later on try to play with some whirldwind on your water with every jar(make sure not too many whirlwind or else i am not sure what will happen to your beta.And also.. watch your finger...)[u can omit this if u think this is crazy and cruel]
+Later on u can start to put two on two fights.(That would be 2vs 2 in four jars)
+Keep the winners and challenge winners against winners(The four winners which 2 vs 2)
+Result will be there will be only one winner.
+Keep the winner and the 2nd place winner and fight against the rest of the losers.
+Of course after that u have to put it to rest(back to the black jars)
+The rest 6 u can try to train them against each other.(like fighting multiple fights( 3 fights for each thrice a week).

Tell me if it works.So far i had trained only 2 betas the most,where the stronger one enjoys nipping other peeps's betas tail and fin.Of course the weaker one i never put it into fight.Because it is only to a train partner for the stronger beta.

*one more thing u have to take note... Betas will lose its confidence once it continues to lose in every battle.In other words it will become weak everytime it lose. About how to let them regain their confidence i dunno much about these*