The Sagittaire is a bad choice for fighting the Clans with unless you're in a city or lots of woods. It's a point-blank fighter that just can't defend itself. Likewise, the Hauptmann Prime lacks sufficient punch at range to really go up against an equal sized Clan mech. Oh, and if that's the original Garm, it's a terrible mech period. Better choices would have been things like the Nightstar, AS-7S2 or AS-7S3 Atlas, HLN-694 Highlander, or Fafnir (any variant) instead of the Sag and Hauptmann, and something faster to replace the Garm, like a Talon or Spector.
Tactics wise, try to limit the Hellstar's ability to park in a protected spot: try maneuvering your forces so that if it wants LOS, it needs to enter an exposed area. If possible, you want to target the Puma and Ryoken early and try to cripple if not outright destroy them. Use as much focused fire as possible- remember, there's no such thing as overkill if the target is still able to move and shoot. For the Mad Cat, taking it at mid-range is probably the best bet: you can't beat the range of its ER Large Lasers and ER ATMs, so don't try. Just keep it out of HE ATM range. For the Burrock, it's going to kill you if you get close, so don't get close. Pound it with every bit of your long-range firepower you can bring to bear on it until it stops moving. Finally, to take out the Hellstar, I recommend just bum-rushing the thing after you've destroyed or crippled at least the Burrock and one or two of the others. Get as many mechs into their optimum ranges as you can and let that sucker have it. Unless you're very, very lucky, it won't go down in one round, but it will go down if you hit it hard enough. You're likely to lose a few mechs doing so, but that's unavoidable. Alternate strategy: bring a lot of mechs with LRMs, at least one mech with a TAG (preferably two, and it's even better if they've got Stealth Armor), and use indirect fired Semi-Guided LRMs on the Hellstar and Burrock until they expire.
