Don't waste your time with that green deck. Large creatures with no form of evasion rarely get anything done.
I don't know what would be "illegal" about a Sliver deck. (Just a foreward, legality means if its able to be used in specific tournament environments). A Sliver deck is in no way legal for the current Standard/Type2 rotation, and mostly isn't legal for Extended (some slivers are, it depends on how long ago they printed specific ones as the rotations happen by blocks, which is the three sets that were made to work together, example being Ravnica/Guildpact/Dissension or ShardsOfAlara/Conflux/AlaraReborn.) I wouldn't say that deck is super powerful other than that you have to remove certain creatures asap. By that I mean truly get them the fuck off the field, because of their sharing ability. Not Pacifism effects.
Red should probably be what you should play. Just keep his guys off the field first. Use your creatures to do damage to the face, and then only aim your burn at his face if he's within lethal range. Play this exactly like chess, thinking turns ahead. Look at your board and think "He has his blockers tapped, so I can get in 6 points right now, I'll drop this guy after combat, he'll swing back with his guys and if I don't block I'll take 7, but that let's my guy live so when he attacks with the others next turn, I'll do 8 more and then throw two Lightning Bolts at him for the last 6. But if, when he attacks with his guys, he decides to play Giant Growth or something to make his guys bigger and possibly kill me, (or even if he holds his guys back to block some of mine), then I'll have to throw the bolts at the creatures in response and then use an extra turn to kill him. That means we'll both have an extra card to shake things up, so let's try to keep him from getting that turn."
EDIT: By the way, that blue deck, due to the nature of blue, is potentially he best deck he has, depending on the build. It will likely lose most games against the red burn though simply because that's a bad matchup for it (like if I'm playing an anti-creature deck and you are playing creatureless combo that kills me through something like a massive fireball in one turn).
I don't know what would be "illegal" about a Sliver deck. (Just a foreward, legality means if its able to be used in specific tournament environments). A Sliver deck is in no way legal for the current Standard/Type2 rotation, and mostly isn't legal for Extended (some slivers are, it depends on how long ago they printed specific ones as the rotations happen by blocks, which is the three sets that were made to work together, example being Ravnica/Guildpact/Dissension or ShardsOfAlara/Conflux/AlaraReborn.) I wouldn't say that deck is super powerful other than that you have to remove certain creatures asap. By that I mean truly get them the fuck off the field, because of their sharing ability. Not Pacifism effects.
Red should probably be what you should play. Just keep his guys off the field first. Use your creatures to do damage to the face, and then only aim your burn at his face if he's within lethal range. Play this exactly like chess, thinking turns ahead. Look at your board and think "He has his blockers tapped, so I can get in 6 points right now, I'll drop this guy after combat, he'll swing back with his guys and if I don't block I'll take 7, but that let's my guy live so when he attacks with the others next turn, I'll do 8 more and then throw two Lightning Bolts at him for the last 6. But if, when he attacks with his guys, he decides to play Giant Growth or something to make his guys bigger and possibly kill me, (or even if he holds his guys back to block some of mine), then I'll have to throw the bolts at the creatures in response and then use an extra turn to kill him. That means we'll both have an extra card to shake things up, so let's try to keep him from getting that turn."
EDIT: By the way, that blue deck, due to the nature of blue, is potentially he best deck he has, depending on the build. It will likely lose most games against the red burn though simply because that's a bad matchup for it (like if I'm playing an anti-creature deck and you are playing creatureless combo that kills me through something like a massive fireball in one turn).

