Deathwatch: Tyranid Invasion tips

I’m currently playing Deathwatch: Tyranid Invasion on my PC after having played it quite a lot on my old iPad (before a patch upped the system requirements to the point that it became unplayable). Here are some tips I have from my time playing it:

• Don’t be afraid to try switching in new marines in when replaying good xp grind missions on Veteran difficulty to quickly level them up. A relatively green recruit with a Tier II bolter can still take a hormagaunt down pretty swiftly.
   
• The Blood Angel special ability, Black Rage, is extremely powerful but very situational. Give your Blood Angels short range weaponry and keep them out of LOS with their team mates so you can use it as often as possible without losing squad mates to friendly fire. Note that with the mission in Act IX when you're guarding the vortex bomb, the vortex bomb can be destroyed by a marine using this ability, causing you to immediately fail the mission. 
   
• As there is no cover mechanic other than LOS, be prepared to use your marines to shield the Devastator(s) when fighting Carnifexes or Hive Tyrants.
   
• Similarly, look out for abilities that allow a marine to reduce all damage taken in a single turn by 90-100% and the item Oath of Immortality. Have such abilities ready during boss fights, so that they can tank an entire round of damage (the AI seems to target the closest marine more often than not). This also means that they can strike without being gibbed by reprisals. Similarly, Nova grenades are also great, as they effectively give you a whole round of shooting at them without fear of reprisal.
   
• Due to the riposte mechanic, positioning an assault marine so that he’s next to a load of low-threat enemies but doesn’t have enough AP left to kill them in his turn can work out quite well, as he’ll probably kill most of them during their turn. If you’re going to do this, trying to stack melee damage reduction is probably a good idea and this approach works best with powerfists (each riposte becomes equivalent to two free APs). 
   
• Because everytime assault marines attack and fail to kill an enemy, there's a chance that they'll take damage, prioritise giving them some form of constant healing. This is where a lot of Blood Angels' marine and item abilities kick in, with the item Rite of Unceasing Service being by far the best. Also, try to soften nasty targets up so assault marines aren't being hit repeatedly. Conversely, remember that enemies that lack a melee attack can't riposte, allowing you to carve into Hive Guard with impunity (assuming that it will die that turn or you've made the marine immune to damage for a turn, given its gun's one-shot kill when starting its turn in its front arc mechanic).  
   
• Think carefully before taking a plasma weapon that can overheat on a marine with the free fire ability. Overheats happen annoyingly often and really cut into the amount of damage free fire can cause.
   
• Speaking of plasma, when you fire a supercharged shot with more than five action points left on your marine, you can (with suitable bonuses) one-shot some powerful foes. Using charge and preparation abilities and some items in combination with this approach can make some encounters that would otherwise gut your squad relatively trival. The use of charge and preparation skills (as can be found on the Tier III Blood Angel heavy marine) can also be used to greatly mitigate the problems caused by overheats (you just roll action points over to the following turn). 
   
• Rather than going for flashy upgrades initially, spend your marines’ XP on accuracy – if they’re not hitting, they’re not doing damage. Once the cost to level them up is equivalent to some of the juicer upgrades, switch to buying upgrades.
   
• Higher Tier weapons are typically less accurate, but Tier II weapons can typically still be used to good effect by rookies. Also a very few Tier IV items have a huge accuracy bonus. Think of low accuracy and and low crit chance as a XP tax. Not all Tier IV weapons are created equal. For example, look at the difference between the Tier IV melta weapons Forge Bane and Firescream. The latter is superior in every way. 
   
• Manage your inventory carefully – frequently prune it to ensure that you can buy new packs of cards as frequently as possible.
   
• Learn which missions are the most deadly – don’t start a mission against Carnifexes or Hive Tyrants when you have a marine just 2000xp off a 20,000xp upgrade – do an easier one first, buy the upgrade and then go in.
   
• At the heroic difficulty setting, a hit that doesn't crit is effectively the same as a miss given the amount of hit points enemies have. Grinding XP to level up crit ratings after you've got your marines' weapons' accuracy to 100% is a good idea. Also consider having multiple +Crit % one off items like Auspexs and some Oaths, so you can take down bosses more easily or clear large numbers of enemies when your squad is danger of being swamped. You may find that once you start fighting genestealers on Heroic, melee weapons that aren't powerfists start to be very inefficient. 
   
• Some of the best buildouts I've ever had are a Lone Wolf with Powerfist and Medusa, the plasma pistol that never overheats, a Blood Angels Sternguard with Firescream (the best Tier IV meltagun) and a Tier IV Ultramarine Devastator with The Doom of Behemoth Tier IV Plasma Cannon. 
   
• The chapter specific +40% damage but -10% accuracy are a double edged sword. I'm convinced that accuracy is lower than displayed in any case. Often it's not worth taking the risk, when lining up a overcharged plasma cannon shot on a Hive Tyrant.