This has been an interesting thread so far. Allow me to throw my advice in here.
Main thing is all the complaints about the Sentinel. Sentinel isn't that hard to play. You just need to stop focussing on how many kills you're getting. Sentinel is for flag defence, and maybe base offense if your cappers are having a hard time of it. For flag defence, kills are easy once you get used to hitting targets. That is the difficult bit, but you'll be surprised how easy it becomes with a bit of practice, and some forethought. Lay some claymores near the flag, you will net some kills simply through that, or at least damage the capper. Then set yourself up somewhere the enemy will be heading straight away from you, instead of horizontally. With that line, it's easy enough to hit them. 500 damage doesn't seem like much, but if you're good you can hit them twice, if not it's enough that the chasers should be able to kill them. (Someone was saying there's no reason to go Soldier over Raider. That's one thing Soldier is good for, chasing a flag capper. The rifle really helps at chipping away that last bit of damage.) Remember, thay should already be damaged, either because of your defenders, or your claymore. More often than not I'll kill an enemy capper in one hit.
As for attacking the enemy base, I wouldn't recommend it usually, but you do see games with 5 or more players sitting on their flag, that's when you go start shooting them. They should scatter, or all rush towards you, leaving the way open for your capper. It's a situational thing, one you'll start to recognise with experience. Generally you should stay, but, for example, if no enemy cappers are getting near (or aren't even trying) and instead the whole team is sitting on their base, by all means go take some shots at them. Lastly, as a Sentinel, if you get a free minute look for a Juggernaut or Sentinel or what have you harassing your base defenses. There is probably one around.
I don't have any more class advice that hasn't already been said, but I will say two more things. Many new players make these mistakes.
First, don't get bogged down with base defence. If you feel like you're unnecessary, i.e. there's two other team mates camping the generator, go do something else. Too often you'll see four players sitting in the gen room. Yes, our generator will be up all game. That's no help if we can't capture the flag because no one is attacking the enemy base or capping.
Second, most vital of all. Do not llama grab. I was playing a game just last night where there was a Brute and a Juggernaut sitting around the enemy base, walking over the enemy flag whenever it returned. Leaving me to zoom over the empty stand at 200+ km/h, then see them drop it seconds later. Do not be that guy. Except for E-Grabs. If the enemy team is about to score, grab their flag. It doesn't matter if it takes them 5 seconds to shoot you down, it may be enough time for your teams chasers to catch up to the enemy flag carrier.
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